I think that whether to use mono/multi repo depends on whether you're willing to dump money into updating everyting at once, or not. If not, monorepos are really a big hindrance. It's better to split on the project boundary (things that may have different development paces), and use git worktree for having different versions of libraries checked out for building/bundling.

It works fairly nicely with meson, as you can simply checkout a worktree of a library into a subprojects directory, and let individual projects move at their own paces even if you don't do releases for the libraries/common code.

It's not really clear why having to update every consumer in sync with library changes is beneficial. Some consumers might have been just experiments, or one off projects, that don't have that much ongoing value to constantly port them to new versions of the common code. But you may still want to get back to them in the future, and want to be able to build them at any time.

It's just easier to manage all this with individual repos.