If too hard to calculate, can you do a quick Fermi estimate? I notice that - assuming there are order 100 billion galaxies in the observable universe - then it seems only a few percent of them are still reachable, even at 99% of c. Already too late to explore and settle most of them :-(

But if someone started 5 billion or 10 billion years ago, would it be more like 100%? (And could they get to more of them at 80% or 90% of light speed, rather than having to zoom up to 99%?)

This could be of some relevance to anthropic arguments. Suppose that early-expanding civilizations tend to end up with much more real estate than late-expanding ones. Then we might expect to be part of one of the early-appearing and early-expanding ones (true under both SSA and SIA), So even if we turned out to be in some sort of ancestor simulation or recreation (a zoo or prehistoric theme park, or something like that), we'd expect to resemble ancestors who lived very early in the history of the universe, rather than 14 billion years in. So maybe this counts as a bit of evidence against the simulation hypothesis?