Agreed, it’s surprisingly difficult to find just one key that won’t conflict in some way. Have you considered Caps Lock, if you don’t use it much? I’ve mapped my right Alt (Alt Gr) to Listary with AHK, and it works reasonably well, but I’ve had to adjust to not having that key in one or two programs. IMO it’s worth it for the convenience everywhere else and (IMO) I find its placement just right.
As an alternative, you could use right Ctrl + right Shift (or any other pair of adjacent keys if you use that combo). Hitting them together overcomes the difficulty finding just one key, but you can do it essentially with one press.