I've had bouts with tendinitis in my top hand over the years. It first started with too much mouse usage from gaming and work. And later it would show up after long shooting sessions. One time it was bugging me for around six months, where I could barely grip my hockey stick. For me, I got under control by doing some simple stretching and strengthening exercises for my wrist.
I'm not a doctor, but when my forearm muscles get tight from prolonged usage, I'm guessing it pulls the wrist tendon taught and they begin rubbing against something like the wrist bones when I move. That irritates the tendons and makes them inflamed. If I don't loosen the muscles, the tendons keep rubbing. By stretching, it loosens the muscles up and stops the the rubbing. No rubbing, the inflammation goes away.
Since I started doing the stretches and exercises, I haven't had a bout of tendinitis, and I haven't changed any of my mouse and/or shooting habits. There were times were I could feel it coming on, like after a couple hours of shooting, but like I said, some stretching and exercise and I was good.
I'm about the same vintage as you, and over the years, as I've gotten older, I've learned that I need to be stretching and doing maintenance work all the time, on my shoulder, on my hips, and my wrists. Etc. It's not the case where I do stretches, the tendinitis goes away, and never have to worry about it again. I have to keep doing the stretches and exercises on a regular basis, usually after exercise, to stop it from coming back. Maybe that's something for you to consider.
Here's a video of some of the wrist stretches and exercises i do. Hope it helps.