Install ncdu, then sudo ncdu -x /var it'll tell you what is taking up space, then if you tell us, we can help you identify how to minimize it and keep it low.
I've never worried about this but I'd use Flatpak. The whole install goes in a specific directory and the metadata/config/data files go in their own specific directory.
I don't believe this is possible however you can quickly filter the current folder you're in with Ctrl + I and start typing. That way if you're looking for The Matrix you can just type Matrix and it'll show it for you.
I love the default Plasma Breeze. I just want to have a usable interface and get work done. But I do LOVE that Plasma can be made to look like practically anything.
No. I run 3x 1440p 144Hz monitors with KDE Plasma on an NVIDIA 1060 GTX, no issues at all.