In my experience, outside Google monorepo (and maybe Meta, don't know, didn't work there) every monorepo just sucks because it's cargo culting.

It's never monorepo. It's one big repo and one or two other small repos (because of permissions, secret code, whatever), copypasted code between repos because people don't set up proper packaging, semi broken builds everywhere because of poor static analysis, and so on.

Unless you are willing to really commit (ha!) to monorepo and all the tooling and devex support that requires, just stick to multiple small repos.