Monorepo is just one small part of the puzzle. If you want to actually achieve the dream state that is alluded to when someone says "monorepo", you have to be willing to endure a super-deep and honest evaluation of your tech stack and processes.

We have been successfully running a monorepo for ~5 years now using nothing more than the bare-ass GitHub PR process with some basic checkbuilds sprinkled on top. We intentionally avoided getting our hands dirty with CI automation as much as was feasible. Building our software manually is so simple its not really worth dedicating a team to worrying about.

I would say the biggest key to our success was finding a way to minimize 3rd party dependencies. By making certain strategic technical choices, we were able to achieve this almost by default. Our repository is effectively hermetically sealed against outside changes in all of the ways that matter to the business owners. Github providing issues at the repository grain is also a very powerful synergy for us in terms of process - we started using issue numbers as the primary key for all the things.

With regard to technical choices - consider that some languages & ecosystems provide a lot more "batteries included" than others, which provides a substantially different landscape around how many 3rd parties may need to be involved and how.