GH will also automatically retarget the dependent PRs to the `main` branch after the dependency PR is merged. I usually just make a note in each stacked PR, "Depends on #N", mostly so that the review and merge sequence is followed. But even this would be clear on its own by the PR number and target branch.

I agree that the PR code review process leaves a lot to be desired, but this particular workflow doesn't seem like a big problem. Though I've never used Phabricator and similar tooling, so there must be something we're missing.