Good morning! So - here is the update on the washer belt. It does seem to have corrected the situation. Here is what we did.
We first installed the new belt - it was much tighter than the old worn one. We did not do anything with checking the chamber pressure port which I thought might have been an issue early on - figured if it ran without getting involved with that, it would be good.
After hooking the water and drain back up, we plugged it in, did the lid lifting reset (not sure it that did anything), and then reset the fault codes to remove all of the old codes in case that would affect anything.
So far I've run three loads, and (fingers crossed) it seems fine. It fills, spins at the proper high speed, drains as normal, and doesn't go into that unbalance blinking spin light cycle. The clothes are much drier coming out of the washer, as they used to be, and I don't hear water sloshing below the tub, and the tub sides aren't visibly wet either.
I tried three different settings and different water levels. The third load I ran was a load of about 6 bath towels. That started to get unbalanced when the rinse/spin started, so I put it on pause and redistributed them, and then it finished the cycle with no issues. I will say that this machine gets out of balance frequently and the redistribution always helped until this problem developed.
I plan on running a few more loads of different items and will let you know if a problem arises again, but for now - it appears that the belt was the solution.
Thanks for all of your help and hopefully this was the fix!