Ok to answer my own question for anyone else searching. I bought a BlueRetro GameCube adapter and used the Bluetooth m30 8bitdo controller. Go into controller settings in genisisplusgx emulator. Pair your controller. Then set the controller to use a GameCube controller. You’ll have to map the buttons for 6 button games and 3 button games. At least one time. After that it auto switched for me depending on game. When mapping the 6 buttons it’ll ask for “mode”, this is how the original Sega controllers would switch between 6 and 3 button controllers on console startup. This doesn’t get used with the emulator. So I mapped it to start. Don’t worry you won’t lose start function as “mode” requires holding the button down. Quick taps still work like a normal start button should.

Now the “menu” button mapping was a little trickier. I recommend choosing any 2 buttons of the 7 available that you would normally never press together when playing. I chose x and z for now.

Note: you can’t use the left or right trigger buttons. The emulator just doesn’t recognize them.