Regarding your shaking mechanic, I personally have tardive dyskinesia, and a substantial amount of my energy goes towards not shaking, so the idea of gamifying a similar motion is not ideal to me. Also, I don't know about anyone else, but sometimes once I start a motion like that I can't stop.
my experience is personal, obviously, and I'm not sure how my condition overlaps with other tremor conditions, but I find m+k easier generally with what I play. Recently I've been using layouts that focus on moving accuracy sensitive presses to the keyboard so the mouse can just be used to point. As an example, in the game war thunder, before i developed TD I had machine guns mapped to mouse 1 and cannons to a side button. Now, pressing that side button while also trying to hold the mouse steady is extremely difficult, so I moved the fire keys to a position close to where I rest my hand on the keyboard.