Haven’t read all the pages but would say don’t buy a 125 – good used ones are hard to find – hire for the test after using a school bike.
What to buy? Perhaps I’ve been spoiled by an old fashioned outlook and forty years of trials competition but, for me, if you want a bike to enjoy then the prime characteristic is that it has to be nimble. Now, this encompasses power as well as weight, handling etc so rules out most sports bikes as, frankly, they’re just too much of a good thing. I’ve never liked fours, too much like riding a vacuum cleaner apart from the weight involved
Assuming you want to keep your licence then you want something you can wind up occasionally without going berserk. It’s no fun having to ride everywhere at a tickover to stay below the limit.
My SV 650 I consider to be on the upper edge of my requirements. The 500 Guzzi Monza was great and the RS 250 Honda perfectly acceptable (apart from the front brake but that’s not a factor nowadays).
I see the 250 class seems to be making a comeback, I’d like to try one but they all seem to be race styled which ain’t for me. A modern version of the RS would be super and well up to passing any traffic on a “proper” road.
Edit, just seen the Transalp reference. My brother did a trackday on his and the guy on the GSXR 1000 couldn’t work out why he was not able to keep up. It’s all about the rider and, powerwise, less can be a lot more.