like you - I am a senior (65) fit and stiff (in all the wrong places). Lot of technical stuff here- but in your post you had the key elements.

"comfort level increased as did RPMs by about 15" - I am sure people here know more than we do- but what I do know is comfort means you can ride longer and farther, higher rpm means less stress on your knees.

Sounds like a winner to me. I just bought a bike and discussed going with a shorter crank- the fitter talked me out of it (172s- old bike had 175s). I think I made a mistake and should have insisted we go to a 170 or 165. Just with there was a way to try this out on a 20 mile ride.

IMHO, go with the 165 - and you have a winning combination.

Len