How can I set a scaled display resolution from the command-line in macOS Catalina?
I’m not aware of any way to do this via defaults write (although it seems like it should be possible). I even checked MacPilot which is usually my go-to for hidden defaults.

My suggestion would be to use display_manager.py which is the best command-line tool that I know of which is being actively developed. It is free, but will require you to figure out the proper settings for your computer.

There’s also SwitchResX which is a commercial app ($16, I think) but if all you want it for is the initial setup, that might be overkill.

Last but not least, there’s ‎Display Menu which is free, but with a $2 in-app purchase will add AppleScript support, which means you can use it from the command line.