Configure keyboard combination to switch apps
I can only think of a way to do it with a single key if you enable the function keys to work without holding down fn.

Then you can assign one of the function keys (F1, F2 etc.) to "Move focus to next window".

First enable "Use all F1, F2, etc. keys as standard function keys like this:


Then assign the "Move focus to next window" to one of the function keys like this: