So it's known that Mate and Cinnamon desktop environments only allow the options "Normal" and "Double DPI" in user interface scaling, unlike Ubuntu which you can change with a slider. These two options on some monitors aren't enough; they require something in the middle. There are several ways to work around this problem and I have stumbled upon one which I have not read about and would like to share it here.
Remember to backup your files because I'm no expert and it's always a good idea to do so.
The premise of this solution is to make the resolution of the monitor higher than its specs so that double DPI will result in a comfortable screen size.
So we'll need to be able to select a higher resolution in the display settings than the resolution of the actual screen.
You can follow this guide. Remember to replace 1600x900 with a higher resolution. I found on my monitor (1920x1080 13.5") 3200x1600 worked well. Then we can select Double DPI in the display settings and we're done!
I also found changing
layout.css.devPixelsPerPx in firefox's about:config to 2.5 worked well with my monitor.
Please let me know if there are any drawbacks to this solution. Currently I'm using this and there are no problems whatsoever other than the HP Driver popup being extremely small. Other than that, I prefer this solution on Linux Mint Mate 19.