> So now we get to the issue: git would want us to rebase every one of these branches individually.

> [..]

> We only get one branch. So what we'd have to do is go back and fix up every single branch here, including our merge.

Not really, sounds close to git's --rebase-merges (formerly --preserve-merges):

  git rebase --onto main@origin main 79d7cf16869f0f293b18ff4fa339d2d053b537c0 --rebase-merges

I'm guessing putting "--update-refs" on top of it would give you exactly what you want. I also have a version without it though.