I am entirely unconvinced that the universal prior really does imply that you should say "substantially less than 1 million".

It seems to me that using the universal prior leads to a probability distribution in which most of the probability goes to hypotheses that take quantum physics seriously, in which case we expect that when you flip a quantum coin a million times we end up with (roughly) 2^1000000 versions of you, almost all of whom see bitstrings with K-complexity about 1 million.

The universal prior should probably also still give some probability to hypotheses in which the universe works non-quantum-mechanically and you get a K-complexity well below 1000000. But not very much.