Unless you are pure OSS or pure closed source - you end up with a poly-repo strategy regardless as you split open and closed code, suffering the annoyances of both systems.
No, what you end up with is a system for mirroring open source code into your repo, and a system for mirroring commits that should be open source from your code into external repos. All active work still happens in a monorepo.
Any good examples of small companies that pull this off and the bots/CI/tool whatever they use to do it?