Thanks: I haven't dove deep enough to know exactly where the issue was, but booting into Gnome wayland session on Ubuntu 22.04 resulted in blurry fonts on my external 32" 4k screen, whereas just switching the session to Gnome on X worked fine.

My search at the time (a few months ago) suggested that subpixel rendering didn't work (changing settings from gnome-tweaks didn't do anything either).