Considering how we made basically no real progress on it mathematically in a whole generation, solving BB(6) within the next decades would be a miracle, and I would bet a lot against it.
I can't see us EVER getting to BB(10), no matter how advanced humanity grows (and it would be meaninglessly large anyway).
I think 765 is just a huge overestimation based on the fact that it is somewhat straightforward to construct.
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.)
