Welcome to this rabbit hole. I recommend the KeymapDB and the other references linked in my keymap repo.

I use a Kensington Expert Mouse for the last 10 years. It has 4 programmable buttons (plus 4 combos) a huge encoder wheel for scrolling and a large trackball in the center. I use it with my whole hand and the biggest advantage is that I just need to move my arm to the left (I use it with my left hand) to grab it. It is pretty fast to switch from the keyboard to the trackball and vice versa. I can even share my left hand palm rest with it. You can see some pictures here.

There is this adapter to use it in a MX switch hole: https://www.thingiverse.com/thing:3958026

Great project. It is really nice to see other 30 key boards. I daily drive a Rommana today and would really like to try one of these. Are you planning to make this project open source?