Hey thanks for sending me down that rabbit hole. I didn't want to use M-Fn since I use that to cycle custom shooting modes, but it turns out the R5 has the option to make it so holding the rear autofocus button temporarily sets a specific focus method. I always use front focus, so now a half-shutter press gives me multi/track, and rear focus + half-shutter gives me single point. I think this'll work great once I get some muscle memory built up for it.
Ahah you're welcome! :-D
Consider that I've the R6 since almost two years, but still today I'm fine tuning the menu options and the buttons mapping.
Having custom shooting modes on M-Fn is indeed interesting, but having the R6 I never thought of it because that function is just mechanical on the mode dial.
What works best for me (weddings/events, and studio/outdoor portraits) is M-Fn switches the AF modes (which as said are limited to just two, so it's a direct toggle), the DoF button switches between Servo and OneShot, the AF-ON button took the function of the M-Fn (and inside it the choice is limited to just the WB, so it's like a WB button), the Star* button toggles ECO on/off, the AF point selection button toggles EYE on/off, the Q button does the Q button but it opens on the exposure meter mode toggle, the SET buttons controls the Drive modes, the joystick does the joystick and centre press it gets the box back in the centre, and the REC button opens the external flash menu.
Front dial change TV, top dial change ISO, rear dial change AV, so the exposure triangle is all at hand with the three dials; control ring is set to exp compensation, but shooting in manual make it basically disabled al, the time.
Seems pretty wicked
but for me works good; apart the exposure triangle, the only two parameters that I change on the fly during a shoot are AF modes and OneShot/Servo, and they're mapped to the two closest buttons to my right hand (M-Fn for the index finger, DoF button for the ring finger), while my thumb moves the joystick to move focus in single point AF or to toggle subjects in multi/track.
Drive/exp mode are available but I never touch them so they're mapped to secondary buttons, and the WB after I set it when beginning the work gets never changed too.