I have the same problem as the OP. I got a 1440p screen, the LG 27GL850, on Big Sur Beta 4.
In Catalina, I solved the issue of small UI by using a tool on GitHub called one-key-hidpi
makes MacOS think a display is a Retina display, making the scaling settings the Retina monitors get show in the Display options System Preferences for my 1440p.
In Big Sur, the tool no longer works since it doesn't have access to the root volume anymore.
Like everyone else here, I discovered that you have to as it is said in:
In the mounted directory, I placed necessary file at /System/Library/Displays/Contents/Resources/Overrides/ to add a display option.
By the way, I used this site: wacky.one/blog/macos-hi-dpi/#one-key to generate the file. The site is in Chinese but the file generation part is at the bottom.
When I rebooted the system, I used a tool called RDM
(download link is at very bottom of site), which gives you more options for display settings to check if the changes took effect, they didn't. However, I suspect it was fault on my part. Perhaps I made an error and put the file into the wrong Library. I will try this again tomorrow, I feel like this method is promising.
If anyone is also stuck, feel free to try this method and post your findings here, thanks.
I will also submit a feedback since I'm on the beta, but it likely won't be addressed any time soon since people have been asking for lower resolution displays to have scaling setting for years now. For now, I will be doing the real work in Catalina.