It's quite inconvenient that screen scaling needs a restart. I come from Unity where scaling is instantly applied. It this not possible in Plasma? I apologize if this is not the right use for the bugtracker.
Under X11 it's surely not possible, that would be an upstream problem. We'll see about Wayland. I'm closing this bugreport since it is not useful for us as an actionable item. Please don't understand it as "devs don't care" but rather as "devs care and that's why they need a clean bugzilla database". :) Thanks for the report, in any case!