I hope that Amazon open-sources Brazil and the surrounding version set ecosystem someday. They're the only large company I know of that uses individual project repos at scale, and they've built tools that solve many of these problems. (I've never worked there, so I don't know how loved those tools are internally.)

Edit: I worked at Microsoft, which also uses tons of tiny repos (at least within Azure). I didn't encounter any good cross-repo management tools, though; apart from having a Jira-like ticketing system built in, Azure DevOps seemed quite a bit worse than GitHub.