You used to be able to make mixer channels narrower, some versions back. I liked that feature, I used it a lot.
You can of course hide and reveal tracks in the mixer pretty easily, but something like what you’re describing with folding tracks would be great. For me the best would be if the folders you create in the project would actually show up in the mixer as strips, like a thin strip that can be folded/unfolded to reveal the tracks.