You may be interested in a recent answer of mine that begins to answer a little part of this question. To crib from the end of it,

One thing is that hooped petticoats have a long history in western fashion, and the more extreme versions developed out of ones that looked roughly like someone wearing many petticoats. I assume the period of interest to you is the 1760s-1770s (which is the segment of this rundown that gets the most attention in film) - by that point, two or three generations of wealthy women had grown up accepting them as a normal part of dress.

Fashions do not come out of nowhere, any more than any other aspect of historical life. Full skirts are still understood as attractive today: couturiers like Oscar de la Renta still use them in ball gowns and wedding dresses, and even the more down-market David's Bridal carries them. We achieve these looks now largely on the backs of seamstresses who came before, who made us of new materials and techniques that stiffen or hold out fabric, but for quite some time, the only way to manage this was to wear multiple petticoats. Full-body images from before the widespread adoption of the farthingale don't show an extreme width, but there is enough fullness there that several would have been needed to make the skirt fall this way. We see something similar in the years before the return of the hoop in the eighteenth century (as discussed in the linked answer). So, the fashion for a full skirt happens, and then someone - in this case, it seems to have been an English person - develops a way to create a larger full skirt with an under-structure. To quote myself again,

Hoops did not come back into fashion until the eighteenth century. Around 1700, some women were wearing heavily starched or glue-stiffened petticoats in order to help hold out their skirts, but by 1710, something rather like the farthingale was back in style. This hooped petticoat was more of a domed shape, and provoked tremendous public comment relating to women taking up too much space on the city streets, flaunting their vanity, etc. etc. In the late 1730s it took on a shape that was flatter in the front and back, but still fairly rounded, which went on to develop into the stereotypical flatness and breadth people often think of when they think of eighteenth-century hoops by about 1750. From there, they became more rounded and narrow by about 1760, and eventually morphed into a bustle situation, with volume at the sides and back, before fading away in the very early nineteenth century.

It looks like the fashion suddenly appeared because the common pop-cultural perception of the entire eighteenth century is, boom, broad hoops from start to finish - but looking at the full progression shows the way that they started as a "reasonable" device and became an aspect of fashion that existed regardless of necessity.

Wigs and hair powder are similar. Longer hair was fashionable for men in the early seventeenth century, and by the 1660s it needed to be so thick, long, and curly that it was much easier to make use of a wig instead. Around 1700, a pale-colored wig was worn with high peaks of hair on either side of a center part; the peaks came down in height over the 1710s, and the color became grey over the 1720s. More conservative or professional men often continued to wear a "bob wig" through the middle of the century, but most fashionable wig-wearing men began to tie theirs back into a queue in the 1730s, with only minor stylistic changes occurring until the return to natural hair in the 1780s-90s. Women, on the other hand, tended not to wear wigs. On parts of the Continent, fashionable women began using powder in the 1710s, but it did not catch on in Britain until the 1750s. Their hair was actually worn rather close to the head until the late 1750s and 1760s, when a slight rise at the top became fashionable, which morphed into quite a tall hairstyle in the 1770s; by the end of that decade, it widened, and then it lowered in the 1780s while still keeping the width. As with men, a more natural, though sometimes still powdered look became fashionable in the 1790s.

Cosmetics I can't discuss in great detail, because I haven't done as thorough a study of it as I have hair and clothing, and to my knowledge nobody else has looked at how these changed over the course of the seventeenth and eighteenth centuries. But it would be a mistake to think of makeup as "caked on". As with big wigs and big gowns, the pop-cultural idea reflects our modern feelings that our standards are ideal and old ones are ludicrous, even though the current Instagram look requires an incredible amount of makeup and time to apply it. Eighteenth-century beauty for men and women required pale skin, but a "naturally" pale look caused by being indoors and having a clear complexion (still quite important), with pink cheeks and lips. Portraiture does not show thick, dead white makeup covered with red blotches. It's probable that - then as now - some people put it on without skill and looked awkward, but the goal was clearly to look "born with it".

Was there any contemporary decenters? Or was it just a minority of the wealthy wearing that fashion?

Fashion could only be fully followed by the wealthy, but there was not a stark line between them and everyone else. Fashion is not just fashion, it is a paradigm for what looks normal to people (particularly before the invention of modern art-couture). In general, people dressed as well as they could afford to. John Collet's High Life Below Stairs (which you can see here) shows the difference between the upper servants like the lady's maid, who dresses about as well as her unseen mistress, and the lower servants, who wear no corsetry and have unstyled and unpowdered hair. Dressing above your station could result in gossip, but dressing below it would do the same. For the most part, people tried to conform to standards for their particular spot on the social scale.

It just all seems so inconvenient, so so so much time and effort.

Convenience above all other considerations is a very modern idea. They simply didn't even think about having a "wash and wear" hairstyle and stretchy clothes that didn't require much fitting, any more than they thought about having gas-powered automobiles or telephones; people who couldn't afford to style their hair beyond the very basics or wear anything but loose jackets or bedgowns were pitied.