> I wish git did is to be able to say that a new branch is based off an old branch (not a commit that used to be that branches head). > Then if I change the base pr in progress (say via rebase or via squashing or the like), I can easily rebase my new commits in the new pr on top of the current state of the branch.

You'd still end up fixing all the conflicts your branch made either way, and also breaking the flow of commits if you didn't update the message. So why bother doing all of that instead of stashing your current project, rebase, and force push it? I believe that's supposed to be the standard approach to handle conflict changes in PRs.