Good thread!
I like how the focus is not on underbiking, rather the right bike for the most technical riding where the OP rides. I’m 50, i no longer enjoy under-biking. I’ve pretty much resolved neck/shoulder and occasional back issues that I lived with for years by finding the right bike for me, for what I ride, where I live. Each of these situations is unique and seems like you’ve found your tool!
Reading Yuri’s post about flat bars was like hearing myself. I’ve tried it 4 different times over many years, the last time I tried 3 different bars on a full squish, each with different rise and sweep, extensions and bar ends, no bueno. Have also tried VO granola bars, better but didn’t like them. I love drop bars!
Getting the right tire that is just good enough on the most technical terrain you face, and as fast on the paved sections as you can muster, will be the ticket. Good luck!