I received the Retro Bit SEGA Mega Drive 6-B - my first official Genesis hardware, new in box - and set it up on the MiSTer. The controller feels exactly like my old 6-button Genesis controller, it feels good and responsive, and the cable is nice and long.
I'm setting an Everdrive-style shortcut Down+Start for the OSD.
I considered using the MODE button for OSD. This button was added to the 6-button controller to be able to switch the controller to a backwards compatible 3-button mode during game startup in the few games that didn't support the 6-button controller, but as it turns out, the MODE button is a normal button, and several games use it for in-game functionality, so it should be mapped correctly on the Retro Bit.
I had to do over 20 fights, this was a bit too difficult to feel fun.