Advanced setup of Dual Monitors with different resolutions

Question

I have a 4k and an FHD monitor and would like to set them up appropriately. By default the FHD monitor is kind of OK but everything on the 4k is too small.

I'm on Ubuntu 18.04, running an nVIDIA Titan V GPU with the 410.48 drivers. I also have no xorg.conf file in /etc/X11.

So, following some answers from across StackExchange and the Arch Wiki, I was using the following method to set up my screens up till today:

1. Go to Activities -> Displays -> Set Scaling to 200%

2. Run the following script every time I log in:

   #!/bin/bash
   xrandr --output DP-0 --primary --scale 1.4x1.4 --pos 3840x0
   xrandr --output HDMI-0 --scale 2x2 --pos 0x432
   

   (Using .xprofile didn't work for me, though that's slightly beside the point.)

DP-0 is my main (4k) monitor, HDMI-0 is my side (FHD) monitor.

This solution worked fine so far, but today xrandr --scale started resetting my Display Scaling to 100%. And what's worse, I cannot change it back to 200% until I reboot. I've also tried changing it back from the terminal but the command

gsettings set org.gnome.desktop.interface scaling-factor 2

again does nothing, and neither does changing that value through dconf.editor.

I rarely reboot my machine (since I need it to be accessible at all times) and I've unfortunately installed quite a bit of software since the last reboot but I don't think any of it had much to do with the X server. How do I find out what's suddenly causing my Display Scaling to reset when I run xrandr? Is there some log file I can look at for xrandr?

---

Additionally, I tried an alternative solution outlined in this answer, but I received an error:

xxx@yyy:~$ cvt 5376 3024 60
# 5376x3024 59.98 Hz (CVT 16.26M9) hsync: 187.80 kHz; pclk: 1409.25 MHz
Modeline "5376x3024_60.00"  1409.25  5376 5848 6440 7504  3024 3027 3032 3131 -hsync +vsync
xxx@yyy:~$ xrandr --newmode "5376x3024_60.00"  1409.25  5376 5848 6440 7504  3024 3027 3032 3131 -hsync +vsync
xxx@yyy:~$ xrandr --addmode DP-0 5376x3024_60.00
X Error of failed request:  BadMatch (invalid parameter attributes)
  Major opcode of failed request:  140 (RANDR)
  Minor opcode of failed request:  18 (RRAddOutputMode)
  Serial number of failed request:  43
  Current serial number in output stream:  44

Any idea what's causing this BadMatch?