But let’s assume you’re a great plasterer, that aside you seem to have no grasp about the human body or mind.
1. Plastering is a skill and nothing more. It can be learnt at any age. With continuous practice *most* people will be able to pick it up and do it at a decent level. While also becoming more productive.
2. Most trades people are poor at best. They lack basic communication skills, terrible organisation skills, minimal business skills, etc. Anyone coming into any trade with half the level of professionalism picked up in a corporate career would absolutely destroy most existing trades people.
I have a much better grasp of mind and body than you can ever imagine, but we're not going there.
So you are basically agreeing with me....continuous practice and getting faster/better over time - which is what I have suggested. But the OP wants only one day a week.
To price a job well takes experience, it's not just a book-keeping exercise, and good plasterers are also organised & good business men - there is a lot of proof of that on this forum alone, but as a newbie on here maybe you haven't seen it.
Whilst somebody might be very fit the change from corporate work to site is harsh, the body has to adapt and nobody, nobody, in their thirties will find that easy. It's a physical job, try to go too fast/too much too soon and the body breaks down.
As an ex-Ultrafit competative athlete and ex-QS & Corporate exec who has made the change, I speak from knowledge and experience. Am I good plaster? No, I'm a patching bodger who knows I could never match or make it on sites, its a sideline issue to my main (stilll physical) line of work. But I like plastering and the Crac on here.