I figured this one out! I have an Asus Q534UX with a 4k display. I noticed that my screen resolution was set to 4k, but I had my scaling set to 2 which makes the desktop usable while still in high resolution.

Once choice is to set the scren resolution to something other than 4k, but that isn't ideal especially when it's not your displays native resolution.

The answer is to do as you've said, copy the monitor.xml file to /var/lib/gdm3/.config and then edit it so you can change the resolution to something more reasonable in the login screen. I changed mine to 2560x1440 but that may be still a bit high. It is MUCH better.

You're only in the login screen for a bit, then when the desktop comes up the resolution and scaling goes to normal.

Save your edits and reboot to see the changes.