Too late! I had to get it working by this morning, and ended up having to put it in DFU mode and then use my Mac mini to do a full erase and restore. Then used Migration Assistant to restore from a Time Machine backup. Other than the normal pains of having to set some things up again and sign in here and there or whatever, I’m just fine right now. But this has to have been one of the weirdest things I’ve had to deal with, Mac-wise, and I’ve dealt with a lot.
After keeping my MBP on and logged in and stuff so things can finish indexing and whatever else happens in the background, I suddenly today get this prompt in the System Settings app!
All I had to do was select Resume Data Sync and then it just went away, no confirmation of anything. I'm not seeing anything missing (that I'm aware of) from my other Mac. Time Machine has been able to continue using my 3 existing Time Machine external drives. Also overall looks like I might have also freed up another 30 or so GB of free space after this whole fiasco.
Still wishing I knew what caused this to begin with. I do use Onyx once in a while, but hadn't used it since being on 14.3.1, and had gone onto the macOS 14.4 betas and NOT used Onyx at all while on 14.4, Beta or release. It was just frozen when I opened the lid in the morning to wake from sleep, and wasn't thinking about WHY it was happening because it was like 5:30am and I hadn't even started drinking my coffee yet ☕😆 I'm totally able to accept that this could have been from my doing, but I didn't have any apps running overnight or anything going that I was aware of. If I would have been smart enough to catch the time on the clock as it was frozen, maybe I could have found more from some logs. Oh well, hopefully this is all past.