I know this is a crazy idea but I say it anyway.
How about putting a maximum number of mutagens you can take for a single class? For example, at most you can have 10 mutagens of a class (or 7 that is a row), after that no more appear.
I think this could help to see multi-class builds again. I have the impression that there are more and more builds focused on a single class that use others only for their auras or specific mutagens.