GitHub allows stacking PRs
you send PR1 to master
and PR2 is a pull request to PR1's branch, the diffs look good, and when PR1 is merged GitHub automatically changes its target branch to master.
EDIT: Ah, from comments further downthread, it seems that the level of stacking enabled is much deeper, as a lot of the manual process is automated. I stopped using Critique over half a decade ago so I no longer remember the details of how it handled this.