It's not as much of a pain if your tooling supports git repos as dependencies. For example a typical multi-repo PR for us with rust is 1) PR against library 2) PR against application that points dependency to PR's branch, makes changes 3) PR review 4) PR 1 is approved and merged 5) PR 2 is changed to point to new master branch of commit 6) PR 2 is approved and merged

Same idea if you use some kind of versioning and release system. It's still a bit of a pain with all the PRs and coordination involved, but at every step every branch is consistent and buildable, you just check it out and hit build.

This is obviously more difficult if you have a more loosely coupled architecture like microservices. But that's self-inflicted pain