I use a full-sized keyboard with a number pad, which might give me an advantage. But, in the preferences for the Preview window, I have Photo Mechanic set to advance to the next photo automatically when the rating or color class is changed. So, it's a right-hand process for me. Fingers on the number pad, with my thumb on the arrow keys if I need to do additional navigation.
(If you don't have a full-size keyboard, USB number pads can be found under USD$10)
Hope this was in some way helpful to you.
-Mick