Nothing should stop you from compiling upstream of the AUR yourself: GitHub - zvova7890/ksysguard6 at kf6
It’s actually pretty light on dependencies. An Arch PKGBUILD is pretty easy to read and should give you hints for all necessary prerequisites and dependencies even for other distros: PKGBUILD - aur.git - AUR Package Repositories
Be sure to build the kf6 branch and not master.