If you specifically want Spot AF (and not the standard single point AF) then unfortunately you can't switch off single point AF in the 'limit autofocus modes' screen.
In this case the best solution is dual back button focus. For events I have my AF-on button set for face+eye detection AF with servo, and then the * button set to spot AF one shot.
If you really wanted to only use the AF-on button just for focusing, you could set up your custom shooting modes on two other buttons, but that means you'd be using three buttons in total.
Depends on your tolerance for toggling and the importance of toggle vs hold, in deciding which is better. I have three options I toggle between right now which works great, but four it still tolerable for me. If the hard toggle is what he's looking for and not holding something down, on a R5/R3 Custom mode button switching is cleanest, but on an R6 Direct AF Method Selection still gives you the hard toggle. To me it sounded like that's what was important, rather than just having quick access through actively holding down a button for multiple AF-On/Registered settings buttons.
Personally, I've stopped using back button focus altogether. It was essential on the 6D with its single cross type point, and any 9 Point Auto-Focus camera. The 7DII and 5DIV were where I was starting to break away from it, but especially on the 5DIV the focus points still were too clustered towards the center. But with the R6 and R5, you can focus wherever you want, initial point tracking with servo eye autofocus is amazing, and the speed at which the joystick moves the AF point is pretty quick and if it's not quick enough I can just touch the screen. It's just extra work to hold down an extra button when the tech has allowed for other easier ways imo.
Obviously it depends on your needs and what you're shooting, but I'm happy to be done with it.
Can't you use dual back button focus? There are lots of you tube videos and web sites with explanations. It isn't in the manual but is "widely" used to switch between spot and eye-AF
The Set button also resets the spot back to the centre point as well which is useful for me.
If you set the joystick to be able to control auto-focus point selection (The only option other than Off), pressing down on it will also re-center the the point. I find that more conveniently placed, and it frees up Set to be programmed to whatever else you want.