jj is actually perfectly fit for this and many other problems. In fact, this is actually the default behavior for jj -- if you squash a bunch of jj commits, the bookmarks on top automatically point to the updated rev tree. Then when syncing the dependent branches to git they all rebase automatically.

The problem however lies in who or what does this rebasing in a multi-tenant environment. You sort of need a system that can do it automatically, or one that gives you control over the process. For example, jj can often get tripped up with branch rules in git since you might accidentally move a bookmark that isn't yours to move, so to speak.