I can tell you with almost absolute certainty that if you have a closed-source repo and need to run CI/CD on a Mac, you will save money in the first month (if not weeks) by buying a mac mini and running the self-hosted actions runner on it.
This was my experience, at least. Mac minutes are crazy expensive.
Okay, do they send you an email to warn you when you're exceeding the free tier
Last I checked the divide was public vs private repos, so no