Ah, I use 3+2 thumbkeys. (Tab, Esc, Space; Backspace, Enter). If I did have to go down to 2+2, I'd put Tab on an easy to access layer.
That said, I think 2-3 is the number of thumb keys that are easy to reach. More than that, and it's harder for the thumb to reach, although allows access to more layers. -- Might as well put tap-hold modifiers on other keys, imo. (e.g. the outer column, or the lower row).
Also.. yeah, Miryoku's layer-opposite-the-thumbkey is elegant, but if it's too restrictive for your usecase, no need to respect it. I'd think e.g. having access to navkeys on the LHS would suit both mouse+keyboard usage, as well as when you've got two hands on the keyboard.
That said, I think 2-3 is the number of thumb keys that are easy to reach. More than that, and it's harder for the thumb to reach, although allows access to more layers. -- Might as well put tap-hold modifiers on other keys, imo. (e.g. the outer column, or the lower row).
Also.. yeah, Miryoku's layer-opposite-the-thumbkey is elegant, but if it's too restrictive for your usecase, no need to respect it. I'd think e.g. having access to navkeys on the LHS would suit both mouse+keyboard usage, as well as when you've got two hands on the keyboard.