You CAN have n different whatever in a polyrepo - that doesn't mean you must. You can settled on a company wide package manager, CI system, build system and whatever. That is what my company has, while each repo has their own setup scripts, there are maybe 100 lines in each repo (and almost all of those lines are redundant and could be combined if I spent some time).

The above breaks down when we have third party code - since they don't follow our common patterns for building so they have to do something different. Bringin that into a monorepo would be just as different from everything else.