My 2 cents: monorepos try to solve what is actually a series of devops problems, by essentially abusing the way an organization or individual uses git (or VCS of your choice). These tools that were never designed for devops in the first place.

To elaborate: I think you are much better off, whether as a team or solo developer, developing small, isolated, and tested / reliable libraries and / or packages that can be combined as needed. Ideally, each package or library can build, test, and ship itself at the CI/CD level. Trying to encapsulate all that into one giant conglomerate of potentially different languages, file types, frameworks and much more sounds exactly what it is: a mess!