Pretty sure xrandr is just for X. Am on debian with Xfce, so not using wayland at present. AFAICT Wayland is the default option for Gnome sessions on debian, but I fell out with gnome around the 2->3 transition and have never looked back.

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 :)