I think we need not more plans, but more experimentation.
It’s hard to reduplicate when things are in different repos, and we just don’t know what sort of abstraction a 3rd repo that NixOS and home-manager should both use should look like. Conversely, with both things in the same repo, we can start deduplicating individual modules and iterative discover what the body of common modules should look it.
Same reason I would want to through NixBSD in the someday and then iteratively deduplicate.