Welcome to the madhouse that is 50+!
Backs are weird, no? I'm 58, and have pretty severe degenerative osteoarthritis in spine, hips, etc etc; along with mild scoliosis -- ugh!
Oddly, like you, I'm still very flexible -- e.g. can still easily touch palms to floor with unbent knees, but can't sustain a forward-leaning posture very long due to back. So, I (now) ride a flat-bar road bike, on the advice of my sports medicine quack. Works perfectly.
OTOH, there are other 50+'ers with back issues who find that a forward/stretched position works best, so tend to prefer full-on road bikes.
Whatever works! Good on ya, welcome again, and keep at it!