What I have found is that riding roads/paths out of my comfort zone over time makes it easier to ride in the comfort zone. I call it “over learning”. For example riding on a bumpy gravel road makes it easier to ride on paved flat roads, riding steeper uphills/downhills makes it easier to ride less steep uphills/downhils, and so on.
For the crotch thing you could try padded bicyle shorts/pants if you haven’t already. It helped me a lot.
For 8 weeks I think you must have had god progress. I was two months short of 60 when I bought my KH 29, and I couldn’t cycle long enough to have a crotch problem after only 8 weeks, and that was without padded bike pants.
What I like to do is find a hill that is just at the limit of or slightly beyond my ability and then I will attempt to climb it repeatedly. I limit myself to six attempts. These are generally short, steep stretches of single track with grades of around 20%. Something like that I could climb without too much difficulty if it were straight and smooth, but when you throw in a few roots and twists it suddenly becomes really hard. It’s a lot like doing windsprints. It really wears you out.