@TheTruth91 I'm referring to this mod:
https://answers.ea.com/t5/Technical-Issues-Mac/Sims-3-64-Bit-Mac-UI-Resolution-too-small/m-p/13443764#M20975
You don't need to make it; someone else already did. The mod's download site has instructions for installing in macOS.
More generally, there are two problems discussed in this thread that are related but distinct. The first is that in newer versions of macOS, the options for in-game resolution are severely limited. The other is that without a mod, the size of the in-game user interface is always measured in pixels. So the higher resolution you use, the less space the UI will occupy on-screen, making it harder to read.
Playing at a lower resolution would be a workaround if that lower resolution were available. That's the point of Better Display: to offer more lower resolutions so you can pick one that makes the UI big enough to read. The mod allows you to instead adjust the UI size independent of the in-game resolution.
Pick the option that works for you and go from there. If Better Display offers you a resolution you're comfortable using, there's no need to do anything except choose that resolution and always play in fullscreen mode. Otherwise, follow the instructions on the mod's download page and adjust the UI to your liking.