Do you think it's useful to consider "epistemic welfare" independently of "instrumental welfare"? To me it seems that approach has led to a number of problems in the past.

1. Solomonoff Induction was historically justified a way similar to your post: you should use the universal prior, because whatever the "right" prior is, if it's computable then substituting the universal prior will cost you only a limited number of epistemic errors. I think this sort of argument is more impressive/persuasive than it should be (at least for some people, including myself when I first came across it), and makes them erroneously think the problem of finding "the right prior" or "a reasonable prior" is already solved or doesn't need to be solved.
2. Thinking that anthropic reasoning / indexical uncertainty is clearly an epistemic problem and hence ought to be solved within epistemology (rather than decision theory), leading for example to dozens of papers arguing over what is the right way to do Bayesian updating in the Sleeping Beauty problem.