You don’t need to build Qt, I just use my distro’s qt, same for the dependencies that kde-builder doesn’t handle.
I have all of Plasma built (workspace component so including apps assigned to Plasma like Discover), plus Dolphin, Kate, Konsole, and a few others. It’s less than 100gb total, including ccache, all the binaries and build directories etc, plus a few months of old logs I could clean out sometime. This is an almost 10 year old Thinkpad, so compilation speed isn’t great, but with ccache it’s ok, sometimes I just need to let it run for a bit (and the initial build is a bit annoying, that one took a while esp. for things like kwin).
I can’t speak to distrobox, I don’t have any experience with it. There’s some instructions on the wiki though. Personally I don’t see a reason unless your distro packages are too old, but I might be missing something - I’m just not a container-minded person.