Trouble with absurd scaling on new Gnome install (Manjaro)

I just installed Gnome-flavored Manjaro on a brand new laptop, which is 14" and has a 2880x1800 (16:10) resolution.

But. The interface is zoomed in way too much, everything’s too big, and in settings I have only 3 zoom levels available (100, 200 (which is the one selected), and 300%). None of them is reasonable (either too much or too little zoomed in).

It gets worse when I connect an external monitor of resolution 1900x1200 — it’s so zoomed in that just the dock take an eighth of the height of the screen…

I’m sure Gnome has been designed to cope with this kind of situation… But how?
Please help!

(found on the Manjaro Forum, appears to work both for Xorg and Wayland).