Enlarge thumb hole and use tape when it shrinks. Or, try switch grips.

From (painful) trial and error, I have all my thumb slug holes drilled a bit large in anticipation of swelling and use a bunch of white tape to get a nice snug fit. In one of the bowling balls I have, I had the pro shop operator replace the slug 4x to get the feel just right, then I reproduced that same oval size on all of my subsequent bowling balls.

For me, I probably use 2-3 pieces of white tape during fall/spring; sometimes up to 4 pieces of tape during winter (Boston, MA). Normally on hot summer days and on the day after bowling 10-15 games from practice/tournament, I use only 1 or 1.5 pieces of white tape.

magic carpet always tore up my thumb.  I use Slowinski's Tiered tape method, works great for me

Enlarge thumb hole and use tape when it shrinks. Or, try switch grips.

+ 1; keep it simple if possible.

12X, your situation is probably pretty common. We have had wide temperature swings here in northern NJ. Hot humid weather causes the thumb to swell. I have not noticed a major change in the other fingers. Your thumb hole should be drilled to the largest size of your thumb. There are lots of tapes on the market that you can use to tighten your grip when the weather cools, and your thumb shrinks a bit.

Vacu-grips on the fingers, I dont know how I made it this far without it.

How do vacu-grips promote easier thumb release?