Shazaam! - I managed to "scale" my monitor to 2560x1440 (you can't choose numbers or display size any more) by moving the text size one size larger in the Display section of System Settings.

I then managed 2560x1440 in full screen windowed mode, and it filled the screen with readable text (still a bit smaller than 1K resolution, but totally workable.

Bonus - I can read the screen better when doing my usual stuff.