The migration is great, but some of the comments in there re: problems, and unexpected wins was particularly useful.
I've always done polyrepos, and they've always had this "thing" I never liked, but couldn't really elaborate. I think it's the silos (and overhead).
I still typically keep my protobuf code in a separate repo, but otherwise, I'm monorepo'ing as well as possible, since modern tooling makes it sooooo much easier to manage