Hi Joel. Welcome.
I’m not a teacher or anything and I’m sure Andrew will have some tips that are better than anything I can offer, but I have also struggled with a sore thumb when playing a lot of barre chords. It’s something that has got a lot better over time, but I still end up with a sore thumb after practicing Moonlight Sonata for a while, for instance.
Apart from strengthening the thumb through practice and being aware of not applying too much pressure (which I’m sure you already know), the one thing that has helped has been making sure my uke doesn’t have a high action. That has meant either sanding down the underside of the saddle until the string height at the 12th fret is in an acceptable range or taking it to a luthier/guitar shop and paying them to do it.
Robin