You can self-host GitLab and have few, if any, incidents that get resolved very quickly. Worked for a company that had no incidents that I observed in ~3 years, now work at a company that had ~2 incidents in 1.5 years.
Perhaps this is a continuing argument for self-hosting, especially if you don't have to expose the instance publicly. But then, if that's an option, you can also self-host GitHub (though I have heard less anecdotes about the stability of that).
I'm confused. You can do zero-downtime backups and replication of databases. I don't know what your company / Gitlab are doing but it seems wrong.