So, if nobody else answers your post, i'll try with my first post here.
I can countersign everything stated from your side: reaching 41 years, I started in last april buying a 26" mtb for the first time after 17 years riding and competiting on the roadbike. Generally, not too bad in shape, but due to a complete different need of muscles and "fast power" for riding trials, shape especially of upper body has improved a lot til now.
Overall it was the best decision ever to start riding trials.
Since then i watched nearly every video from AliC, Duncan Shaw and a lot of others. Really fantastic tutorials besides of the basic trashzen stuff.
So with improving skills on the rearwheel, the need for a real trial bike was there. So since October a used Monty kamel 220 double disc helps me to progress much faster than my heavy low BB MTB allowed me to. Ok, looks a bit strange with a 1.9m guy riding it...
So my advise is to buy a more trial specific bike than a bmx or usual mtb, if you want to improve quickly.
Actually i'm working on frontwheel hops. But also Sidehops need some training to get it done smooth and stable. 25 inch are the limit actually.
The only thing i'm a little bit proud of is my improvement in bunnyhop-to-backwheel. Never ever expected to hop up 30 inch to the backwheel when starting last year and had problems to get up onto a kerb. But once you get familiar with the combination of pulling and jumping off as fast as possible, it works nearly on its own.
Other thing which helped me was a change from a high backsweep/upsweep trial bar, to a jitsyhigh rise street bar with no upsweep, which feels much more natural to me and enables to get mire force onto the bar.
Have a nice weekend everybody...
...and here two videos from the last days and a compilation since okctober from my own video analysis.