MrSteve- Highly recommend you check out Foundation Training, created by a surfer/chiro. Really simple, intuitive stretches that work for back pain and alleviates strain from sitting all day/bad posture. Can do on your own or find a local trainer.
https://stream.foundationtraining.com
Volume seems about right, but I agree another inch or two length might help. I'm 40's, 178lbs, 6ft tall, and ride a 5'6" and 5'8" keel fish too. The 5'8" really doesn't feel much bigger, but does make a difference in paddling /catching waves. You might also try a quad fish rather than twin keel (for hold on those late bottom turns), if you haven't already. Since you are used to shorter shapes and have had Tomos before, I've had good experience in the waves you describe on Tomo's Omni (never had a bad session on it) and Cymatic (fantastic for late drops), at 5'9" and 5'8" respectively.
Also, my 5'9" ASQ is the best paddling alt board I've tried, and gets into waves super early (no late drops)... but I've never really surfed it in dumpy beachbreaks.