yess this worked (really had to add that modeline, there was no default 3840x2160 30Hz mode unlike on my other laptop), so much better now 
(btw i had no ideaaa it was so easy to switch to X11 by just logging in and clicking the gear icon and selecting the X11 session)
It’s just, right now, moving any window to the other screen results in ludicrously large fonts because of the resolution difference.
edit: looks like GNOME on X11 doesn’t support per-monitor scaling (if you change scaling for one monitor, the other changes too), so comparatively everything still looks huge on the other monitor, but it’s better than before