I don’t know much about the history behind wealthy women’s duties post-marriage, but I am a literary snob, so I wanted to clarify some things about your assumptions about the Bennets. Namely, Elizabeth Bennet did not grow up poor, or even humble.

Her father is a landed gentry. And though he might have “Esquire” after his name, that was a title of the gentry, and not -as we assume now- a lawyer. His income was £2,000 a year, which would work out to be £160,000... but in fact, he actually had more purchasing power than someone with a similar income today, as he had five full-time servants (butler, housekeeper, cook, maid, and scullery maid) for a family of seven. No one making £160,000 today could afford to pay the wages of that many people. Also, that income doesn’t include the house and estate, which had no debt or mortgage on it.

The reason why the Bennets are so desperate to marry their daughters well was because Mrs Bennet was a spendthrift. She spent the entirety of Mr Bennet’s income every year to it’s end. Mr Bennet assumed he’d have a son in his early years of marriage, and valued peace over savings, so didn’t bother to fight with his bride over reining in her spending. After it became clear that he would only have daughters, the habits of the marriage had been set, and he resigned himself to his wife’s lack of economy, rather than fighting to save a little. Thus, the girls have little dowry to speak of.

Had Mr Bennet had a son, Longbourn House (which was not a mere house as we know it, but an estate manor) would have been passed to him. As explained in the book, the estate was entailed, meaning it must be passed to the next living male relative, and was not something that could be set aside for any daughter’s dowry. Since Mrs Bennet bemoans her “impending” poverty upon Mr Bennets eventual death, we can also deduce that the entirety of the family’s income comes from rents from the estate. Also indicating that Mr Bennet made no investments to try and grow his wealth, or save something over for a rainy day. However, if one of the girls marries and bears a son, then that son could inherit Longbourn House instead of Mr Collins. Which is why Mr Collins visited and proposed to Elizabeth; if he could secure a daughter, then he wouldn’t have to lose Longbourn when Mr Bennet dies, but hold it in trust for his own son to later inherit.

Where we may see some disconnect in how much they actually have and how much we’re told they have (often from Mrs Bennet) is because Mr Bennet had taken great pains to protect his family from debt. A woman capable of spending £2,000 flat her first year of marriage would definitely consider the lack of inflation of her funds “poverty” as she bore five children that required clothing, entertainment, and incidentals as they grew.

Another issue at play isn’t about wealth, but about class. Mrs Bennet is not from the aristocracy, nobility, or gentry. Her father was a plain old Mister, and a working lawyer. As I understand it at the time was not how we think of an attorney nowadays, who has to pass the bar (a barrister), but was more like a solicitor. Marrying Mr Bennet was a coup, of a sort, for her, and gave her the impression that she was a bigger deal than she actually was. In the company of her husband’s peers, she often behaved shamelessly, and spoke openly about matters (for this example, financial matters) that others of her adopted class would never have dared to mention aloud. This sort of “drags down” the impression readers have about the family’s wealth. And, considering the mother of the house was supposed to educate her children on the nuances of running a home, your question of whether Elizabeth would have been capable -much less comfortable- in running an estate the size of Pemberley is a valid one. Especially since they did not hire a governess for the girls, much to Lady Catherine de Borough’s dismay.

Also contributing to this idea that the Bennets were some kind of poor relation, the 2005 film chose a simple, hand-me-down aesthetic for the Bennet family costumes, whereas the 1995 miniseries does a much better job of dressing the family in the silks, muslins, accessories, and fripperies the Bennet girls were accustomed to. Especially for the older, out, and marriageable daughters. If anyone was going to wear old, or out-of-fashion clothes in that family, it was going to be Mary, Kitty, and Lydia.

Mr Darcy’s income was £10,000 a year, which is five times that of the Bennets. (In today’s world the incomes would be like your cardiologist’s daughter marrying the grandson of a Fortune 500 CEO; it’s not that crazy a difference, in the end.) However, it is their class difference that is the foil in the novel (Mr Darcy being the grandson and nephew of an Earl, Ms Bennet the daughter of an Esquire, which ranks below Lady Catherine’s husband, who was a knight, but she gets to keep the rank of her birth in any event), not their incomes. Americans tend to think income=class, but that’s not so. Had the Bennet’s income been Mr Darcy’s equal, it still would have been a set-down in rank for Mr Darcy to marry Elizabeth, and really only explainable if she had some with a sizable dowry - which she didn’t.

Her family could have been poor as church mice, and it would have been easier for Mr Darcy to integrate her with his social class if she had been of greater or equal, or, perhaps, even a single step down in, rank than him. Now, she’s not a commoner, but had they been a real couple, they probably would have had problems with his peers. We assume that Elizabeth, and their great love, would win over any but the nastiest of gossips.

Edit: Oh, yeah, my source is the OG source material: Pride and Prejudice by Jane Austin, read at least 2x/year for twenty years.

It's important to note that Elizabeth Bennet didn't really grow up in a much more humble home. With an income of £3,000 a year, the Bennet family was extremely wealthy by most English people's standards. (Even by other Austen heroines' standards! The sensible Elinor Dashwood considers £1,000 a year to be a very substantial income.) There are two things that make the Bennets come off as relatively humble: the dowries set out for the daughters in their parents' marriage settlement, and the culture of their family home.

The latter is largely due to Mrs. Bennet's premarital social position (as well as the family's habit of spending the entire income). While Mr. Bennet comes from inherited wealth - Longbourn has been passed down in his family, along with the massive fortune that passively generates this income - Mrs. Bennet was born in the middle class. (Her father was an attorney in Meryton, like her sister's husband, and her brother, Mr. Gardiner, is some kind of retailer with warehouses near his Cheapside home.) There are a number of aspects of the plot that are ultimately derived from this backstory.

Obviously, it affects the opinions Mr. Darcy and the sisters of Mr. Bingley have of the Bennets. It also potentially explains why the daughters' dowries are so low: they were set by Mr. Bennet senior, who might have been making a statement of displeasure with the match. And it's reflected in the way that Mrs. Bennet, who as the mother was in charge of determining her childrens' education, neither hired a governess (a type of employee she would have had no experience with from her own upbringing) nor raised them to have the type of knowledge expected for their level of society (because she didn't grow up with it either). Jane and Elizabeth turn out as "ladies" because, as was literary tradition at the time, they have innately good characters and chose to work on themselves, but even so they aren't particularly accomplished, and of course Mary, Kitty, and Lydia have no restraint in their own ways. But in other respects, Elizabeth ends the book prepared for the basic expectations of being a wealthy married woman. Her responsibilities as Mrs. Darcy would mirror Mr. Darcy's own:

"He is the best landlord, and the best master," said she, "that ever lived. Not like the wild young men now-a-days, who think of nothing but themselves. There is not one of his tenants or servants but what will give him a good name. Some people call him proud; but I am sure I never saw any thing of it. To my fancy, it is only because he does not rattle away like other young men."

Mr. Darcy's main employment is taking an interest in his estate and his tenants. He doesn't have to be involved in the day-to-day work of it - he employs a steward, who would be in charge of supervising and directing work being done on the grounds, and dealing with tenants and collecting rents - but he sets the overall policy, and clearly does so in a very benevolent way, while also keeping up some kind of light personal relationships with them. The ideal landowner of the time was able to condescend to help out his tenants from time to time, offering them gifts of game or employment as needed. Likewise, Mrs. Darcy wouldn't spend her time arranging the village fetes, but would act as a kind of "patroness" of the village, its church, and its school, and would also need to meet the tenant families in order to know which needed charitable gifts of food or clothes. And likewise, the bulk of the housekeeping work would be done by the housekeeper (managing the linens and laundry, instructing new servants, and overseeing a lot of food preparation/preservation), but Mrs. Darcy would set the policy and instruct the housekeeper if there were changes to be made. We don't get a great sense of how Elizabeth does in this regard at Longbourn because it's simply not a part of the novel, though we do know that she grew up wealthy enough to not do anything in the kitchen, as Mrs. Bennet snaps at Mr. Collins - whereas in Emma, Emma's charitable work and household management is part of the plot - but while the scale might be a little overwhelming due to the grandness of Pemberley, she is probably reasonably well-equipped.

Her greatest personal duties would be more social. She would have the highest social standing in her village, and in the broader surrounding area unless there was someone titled who lived there, and would therefore both be invited regularly to parties and be expected to throw her own. This wasn't just about having fun, but being shown respect and showing respect in return, and keeping social links between her family and others active - which would also require regular correspondence and calls paid in person. Here we can definitely say Elizabeth is prepared, since she's shown to be very socially adept and comfortable talking with anyone.

In addition, mothers were (as I wrote above) expected to be involved with the education of their children. By the end of the eighteenth century, the idea of a motherly instinct to nurture was assumed to be universal, and the idea that mothers were inherently good was coming into being. A woman who had children was in charge of teaching them or having them taught both ordinary subjects like reading, mathematics, languages, and dancing, and of inculcating a moral sense - it was a mother's job to make sure that her children grew up into good people. As mentioned above, Elizabeth herself never had a governess and her mother didn't pay much attention to her education, but books and masters were provided when she asked to learn; form the text we can tell she was at least taught to read, dance, play the piano (even if she never cared enough to practice), and sing. While the Pemberley nursery would probably be fully staffed with nursemaids to take care of the future Darcy children's bodily needs, Elizabeth would come in daily to look after their minds and morals. While she would probably be good at the moral side of education, she would most likely hire a governess to handle the more academic subjects and accomplishments, and if she had sons they would need to be educated by a tutor at some point to prepare them for university.