If you make refactor PRs as you go, do you end up merging redactors towards a dead end and then--once you realize it's a dead end--merging even more refractors in the other direction?
I usually wait until I have the big PR done and then merge redactors towards it because then at least I know the road I'm paving has a workable destination.
This is why I design the heckin' huge change at the start, and then cherry pick the actual change (and associated tests) into a ton of smaller PRs, including "refactor here", "make this function + tests", "make this class + tests", "integrate the code + tests", and so on, as many times as necessary to have testable and reviewable units of code.
If I went about and made a ton of changes that all went into dead ends, honestly, I would get pretty demoralized and I think my team would get annoyed, especially if I then went through and rolled back many of those changes as not ending up being necessary.
I usually wait until I have the big PR done and then merge redactors towards it because then at least I know the road I'm paving has a workable destination.