So after much playing about, I have found a way that works: set Position and Size to “Remember” as well as Ignore requested geometry to “Force” - but it has to be replicated manually for every application / window I encounter that does not behave properly by default.
So far most non-system windows (e.g. Firefox, Discord) behave properly by themselves but every system thing (e.g. settings, dolphin, terminal, software store) need this frustrating fix applying. Notably, while main Firefox windows are fine, devtools also needed these settings applying to it. Odd.