Well, you don't need to run a sound server, although no sound server means the volume widget won't work. And you don't "run Wayland", you run KWin bare (instead of on xorg) and it provides a Wayland interface - it's actually lighter (and arguably has a smaller attack surface). As for D-Bus, it helps provide all the functionality that makes Plasma, Plasma. But you don't have to use it. I have no idea why you'd want to neuter Plasma until it provides no more functionality than twm (just run twm?), but if that is your sincere wish, you need only run KWin standalone.

It runs great on FreeBSD, it's my daily driver. Wayland is not needed (and currently not working on FreeBSD with KDE anyway), but D-Bus unfortunately is. Sound you can do without of course if you don't need it.

Do you mean LXQT then?