I've actually written several answers about the transitions in men's fashion at the end of the eighteenth and beginning of the nineteenth centuries and Beau Brummell's place in that transition, which you can find in the Origins of Modern Menswear section of my profile. I also wrote a Twitter thread about it. To mix it up and since I've already given the essential answer (which is that no, this was a much broader trend and there is no sudden break with fanciness), I'm going to go through the thread and respond to some of the points that I haven't really addressed in any of the previous links.

Dandyism was a whole aesthetic movement -- you know how in the 80s/90s there was that whole trend of "cynicism is cool"? The cool guy in shades at the party, lounging with studied carelessness against a doorjamb and smoking?

The dandies were more the spiritual successors to the macaronis and other 18th and 17th century stereotypes of young men with an interest in fashion; while the fashionable young men of this period wore more subdued colors than the macaronis and no wigs or hair powder, they would become viciously mocked in the press and the print-shop satires for supposedly wearing corsets and padding to achieve a proper silhouette, and for their high, starched collars and tight coats that supposedly made it difficult for them to move. They viewed themselves as extremely cool, but they were not actually looked up to by all of society.

Something else I should mention here is that while "dandy" had been used since the 1780s in slang (usually in the phrase, "that's the dandy!", meaning "that's a fine thing"), it didn't really start to be applied specifically to a social set or group of men until the mid-1810s. Despite the scenes in the excellently fun movie Beau Brummell: This Charming Man where Brummell presents "the dandy" as an alternative to "the fop", this was more of a sarcastic nickname that was applied to fashionable men after his fall from grace in the early part of the decade. It was not the case that they formed a "movement" and gave it a name; they were just men dressing fashionably, and when the press wanted to look down on them, it just referred to them as "men of fashion". I'm trying not to belabor this, but my point is that Brummell didn't go, "I have a new plan for how men should dress," and then they gave themselves a name to distinguish themselves from other fashionable men - they just were the fashionable men. There is a great moralizing article in the Scots Magazine in 1808 titled "Desultory Remarks on Fashion", condemning the behavior of fashionable men, that makes it clear that the issue was being fashionable in and of itself, and not being part of a movement.

BFB was not a gentleman or an aristocrat. He was just Some Guy. He was Some Guy so f-----g hard that IN HIS LIFETIME he wrenched men's fashion in a totally new direction.

Brummell was a gentleman, from an elite though not aristocratic family. His father was a politician and the owner of a large Gothic mansion and estate called Donnington Grove; he was sent to Eton and then attended Oxford. This background was a big part of what was required for gentlemanhood at the time. In the early 1790s, he purchased a commission in the Tenth Royal Hussars, a fashionable cavalry regiment associated with the Prince of Wales, the future George IV, which was also a super gentlemanly thing to do. The Hussars were an expensive place to be. Rather than it being a flashy job where you got paid a high salary to sustain your lifestyle, you were expected to have extensive family money to maintain your very fancy uniform (which cost twice as much as a normal cavalry officer's, and more than four times as much as an infantry officer's), pay for your horses, pay for your wine, pay your gambling debts, pay your prostitutes, etc. Brummell's father died in 1794 and the family estate was sold in order to equally partition the money between his children; Beau received £30,000, and between paying for his commissions and financing his lifestyle, it did get eaten up to some extent. But he'd had the money and he was not a penniless nobody who rose from nowhere.

See, he couldn't afford fancy jewelry or expensive fabrics, so he made it unacceptable for ANY MAN to wear jewelry or expensive fabrics. He convinced everyone that it was vulgar and effeminate.

This is simply wrong. Brummell's personal style - which, as I point out in those earlier links, was derived from the prevailing fashion rather than vice versa - relied on well-made clothes from the best materials, always kept in the best condition. This was not cheap, in a time where materials were much more expensive, proportionally, than they are today, and particularly when you habitually purchase the top-of-the-line superfine wool and cashmere for your coats, the finest muslins for your cravats, etc. If it had been fashionable for men to wear jewelry beyond watch-chains and stick-pins, he would have worn the best, but it wasn't, so he didn't. While he had to cut back on most other expenses, an anecdote (that could be false, but it's the impression that counts) has him stating that he thought £800 an economical amount to spend on clothes per year - this was a decent year's income for an upper-middle-class family at the time. Rowland does come around to this eventually in the Twitter thread, but it's in direct contradiction of her original point, that he just happened to dress plainly because he couldn't afford gaudier. He could have, but he didn't want that because it was not in the least fashionable.

There is a statue of BFB on Jermyn Street in London, with the inscription: "To be truly elegant one should not be noticed." He's THE LITERAL AUTHOR OF THAT PART OF TOXIC MASCULINITY. He is the inventor of the reason that many men today feel self-conscious in anything eye-catching or unique. Shapeless hoodies and jeans or cargo pants for everybody! A suit that looks exactly like everyone else's!

This is more of a misunderstanding. Brummell's point (if he really said it; this is another line that's been attributed to him rather than being something we can find in his own writings) is that an elegant man's outfit should be totally in harmony with itself and in line with current fashion, and so well-made that the average viewer looks right past it. He would have been disgusted by the idea of wearing shapeless off-the-rack clothing that hid the body. He didn't believe that all men should wear suits that looked like everyone else's: he believed that all wealthy men of fashion should wear bespoke suits that fit them perfectly, which means extensive customization and, inherently, individuality within the acceptable parameters.