My xrandr line is not that easy to read but effectively, after I've set xfce hidpi support to double the size of screen elements, I increase the view window of the smaller screen by a factor of 1.8, and of the larger screen by a factor of 1.2, and put them next to each other -
xrandr --output HDMI-0 --mode 1920x1200 --pos 0x100 --scale 1.8x1.8 --output DP-2 --mode 3840x2160 --primary --scale 1.2x1.2 --pos 3456x-180
If Wayland can do that without so much messing around, great :)
On X11, scaling of entire framebuffer (i.e. scaling macOS style) is a standard xrandr functionality, but it was never exposed to UI. It has some drawbacks that macOS never had to solve (how it behaves over remote x11/vnc/etc, or memory requirements for framebuffer, for example), that make it unsuitable as a general solution. It is basically one of the hacks complained in this threads.
Are you saying it would have worked had I used xrandr?
Are you sure you were running Wayland? In Fedora under Wayland, you could pick a monitor and choose scaling for it in the setting applet.
It is quite possible that 18.04 has some bugs. I don't have 18.04 machine with hidpi monitor at hand, so I can't have a look myself, so I chalk it up that there is something weird happening.