You shouldn’t need the Qt5 packages with plasma 6, you should be able to remove them.
I arrived here because of the same issue with the breeze theme but it has been fixed already in this PR. You can remove kdePackages.ksvg and it should all work again after an update. Welcome to unstable 
As for sddm theme in plasma settings, you need the kdePackages.sddm-kcm package installed. Just add it to environment.systemPackages.