Didn't see your post until I put up one of my own with the exact same issue. I'm also on T-Mobile, using a Tab S7 and a Galaxy S20+ phone. I've tried everything you have - and more to no avail. I guess the only positive thing is seeing someone else with the same problem, so I know it's not unique. If you figure out a solution, please post!
I turned off the tablet overnight and restarted it this morning. No change. Messages are still sending, but do not show as sent.
I found a fix that seems to be working for me. I switched from the stock Samsung messaging app to the Google messaging app. I had to link the two using a QR code, and now everything seems to be working. (At least after one day.) ... FWIW, I also discovered that multiple text messages were not coming through to me, with no rhyme or reason - a problem that started around the same time my tablet began acting up. (I found a fair number of threads with T-Mobile customers having the missing text issue, too.) ... Though I can't understand why, maybe all of this had to do with the Samsung stock message app. Or at least I hope so.
I ended up doing a factory reset on the s5e. The first date restore was from a backup of the s5e. The problem persisted. Second days restore, I used data from my phone. That somehow solved my problem. I still don't know what caused the problem on the first place, but because the s5e is really just used for my entertainment, and everything was backed up, pictures videos etc., if we really just an annoyance to do the factory reset.
Good luck to anyone else with this problem.