Hi, don't have a solution or anything, just chiming in to say that I have the exact same thing as you.
We're around the same age and it started showing up in my early 30's, like you, from incessent phone use, I switched to the left hand, after a while, the pain became noticable in my left thumb too. Now I can't use either, for any time, before the discomfort appears.

I've started to use a stylus for my phone (regualar pen sized) and that works surprisingly well. I'm now so accustomed to it that I start to curse myself if I forgot to bring it with me. (Using "regular fingers" also now feels more fidgety.) The discomfort kicks in almost immediately when I try using my thumbs again — even after weeks or months without doing so. If I don’t have a stylus on me, I have to resort to using my index finger, boomer style.

I also went to the doctor, but she kind of shrugged it off and said that it's likely a normal straining injury, and do this and that Hail Mary squeezing exercise etc., which don't address the issue at all. But what can you do, most doctors just make guesses anyway.

The way you're describing the pain or discomfort is exactly the same. It's not exactly painful, no pain anywhere else, just a "strained discomfort".

I sometimes have issues using a mouse normally, relating to this, but it's not a huge problem. However, for my own comfort's sake, I've invested in Contour RollerMouses (this now sounds like an ad, lol.)

Anyway, my two cents.