[Sniffnoy]

First off, it's not Collatz, it's a Collatz-like problem.

But yes, I'd be very surprised if Collatz were actually undecideable, even if it's well beyond the reach of current mathematics.

I agree with your statements about BB(10) and BB(6), but they just aren't very relevant. I agree those involve extremely difficult problems likely well beyond the reach of current mathematics, but I'd still be very surprised to find anything undecideable in there. There's a big difference between being truly undecideable and merely well beyond the reach of current mathematics!

(Also, the current record for undecideability is 745, or 765.)