So I worked on this on a whim in a random thread some weeks back.
I revisited it recently and cleaned it up a little.
Its a little script that just automates what I would do for HiDPI config in KDE Plasma.
All you really need to use it is which DPI you would like.
It is largely intended for those who have not already made modifications to files such as /etc/environment or /etc/sddm.conf and so on.
(though the script could give clues for the curious as to which files and options can be useful)