I can't downscale from 4K (monitor resolution) to 1080p, or anything. The slider seems to not do anything. I have a feeling that this might be due to Minecraft not knowing how to talk to Wayland.

Most Linux distro's are now using Wayland by default now, so this is a bit of an issue https://www.ubuntubuzz.com/2021/10/distros-which-adopted-wayland-in-2021.html

Of course, I could be entirely wrong and it could be nothing to do with Wayland, maybe it's to do with the fact I'm running the launcher in a flatpak - which I will test soon and edit this issue.