I also have arthritis in my right thumb knuckles. My doc gave me a cortisone shot and I take 650 arthritis strength Tylenol…2 in the morning and 2 in the evening. Seems to help. I first noticed it last fall when I started my bowling season. Thumb swelling went down but is still stiffer than it used to be.
I miss working on ooutboards too being in a wheel chair from covid & west nile. I remember changing out a 60 circuit electrical distribution panel at dairy queen & my hands cramped up & couldn’t hold a screwdriver. Had to finish cause the ice cream was melting!!! Life as a sparky!! 🙂
I have a cordless Dewalt impact driver I mainly use to remove flywheels but have occasional used it on stubborn screws (with caution so as not to strip the screw head). While I don’t have one perhaps a gyroscopic screwdriver might help as well. Perhaps a couple of power tools would help to reduce the stress on you hands.
.
.