I worked at a place that was allergic to contributing patches upstream. We maintained a lot of internal forks for things and had no problem collaborating.

You don't need to push the rebased branch to the same branch on your remote, if that's an issue (although I don't see how it is).

Maybe this is a case of "Dropbox is just rsync", but I feel like just learning git and using it is easier than learning a new tool.

> I feel like just learning git and using it is easier than learning a new tool

I would agree if this "new tool" we're talking about wasn't just a simple wrapper over existing git commands. You can learn it in its entirety, including how it works (not just how to use it), in a matter of a half hour or less.

We do this for some of the components that are shared between Servo and Firefox. Firefox is upstream, and on the Servo side we have automated and manual syncing. The automated syncing mirrors the upstream `main` branch to our `upstream` without changes daily. The manual syncing rebases our changes on top a new upstream version through a manual rebase process. This happens monthly and each sync is pushed to a new branch to maintain history.

Between monthly syncs we push our own changes to our latest monthly branch (which also get manually sent upstream when we get a chance).