Sorry you have joined the club ..
But welcome to the club.
As for tendonitis...
I think I spent my 50's dealing with endless aches & pains.
One of which was diagnosed as tendonitis of the wrist
Also had a touch of tennis elbow, without the tennis ( felt robbed )
Whether coincidental or not all those symptoms ended once
A. I reached 60
B. Began low carbing
My monies on LCHF personally.
It does seem once we settle into it, low carb does help a lot of ailments...(asthma here, and others say arthritis eased )
Doesnt help you, I'll admit
But maybe worth keeping in mind over the months ahead to see if it does ease.