Other comments mention VSTs; they work, but are typically Windows only and I suspect more on HN use macOS.
Elgato Wave microphones (https://www.elgato.com/en/wave-1 , $100/$150), which are good quality for the price, allow access to the Wave Link software which can be used to mix audio before piping it into OBS. A recent update allows users on macOS to use the native Audio Units (including equalizers/gates), and offers an audio stream that's just the added effects for easy use in other apps. https://help.elgato.com/hc/en-us/articles/4416040077069-Wave...
Alternatively, you can get Loopback for $100 (https://rogueamoeba.com/loopback/) for mixing or Soundsource for $40 (https://rogueamoeba.com/soundsource/) for AUs both of which I believe is what is what Wave Link uses regardless, so might as well get a good microphone out of it if you do not have one already.