I'm not sure what FEX could offer on macOS that Rosetta 2 doesn't already, with better performance thanks to Apple Silicon magic.
Running x86 code on ARM macOS is the most solved part of the stack, if anything needs work it's the API translation layers.
Rosetta 2 is going to be EOL'd within the next few years. A more permanent solution would certainly be welcome.
Aren’t most Mac issues now around Metal vs OpenGL and DirectX?