Gitlab has that fine-grained control feature, I think. I understand
that managing accounts and permissions will take some time, but maybe
this can be delegated to someone else you trust?
The largest part of the work would be the coordination, not the work itself.
In addition, I don't see how having to figure out what branch the work is on would be easier than figuring out which repo to go to. And authors who want to advertise separate repos would still do so. It just doesn't seem like a benefit.
I'm quick at keeping up with drafts, so no one has to wait long, anyway.