I have a Prusa MK3 with the MMU2, and I definitely plan on upgrading at some point to the MK4 and MMU3. Since I don’t have any direct experience with any multi-material options other than the MK3 with MMU2, the only thing I will say is to ensure that whatever you buy is an open source printer. I cannot tell you how much of a difference that makes when there are so many incredibly smart makers in the community that can take full advantage of that to provide solutions for basically any problem. My first printer was closed source, and I had a document that was several pages long that broke down everything that you needed to do (and not do) to get that machine working well and upgrade it yourself within the confines of what was possible. Prusa support is top notch, as is their quality. I can’t recommend the brand enough, even if I don’t have experience with the latest multi-material options.
I’m definitely surprised to hear that you’ve had so many problems with your Prusa. Mine has been smooth sailing pretty much since day one.