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?

My layout is designed for 30 keys, but two of them are completely replicated by combos, and the 4 alphas in the center columns are also replicated in the secondary alpha layer. What it means is that I can type with my layout using only 24 physical keys: 2 per thumb, 1 per pinkie and 3 for each of the other fingers. For the other layers, I still need the center columns, but it is a great releaf to be able to type any word without any finger stretch.

HDMI cables, with 19pins (I will use it in my next build), or flat cables, which you can find in different sizes and pin counts (I used some with 10 pins in my Rommana). Both types of connector and cables can be easily found in Aliexpress.

I use a custom layout that is way different than qwerty, in ortho and column staggered boards, for about 2 years. And for my surprise I still can type as fast as before in my laptop. If you go for a layout that is not similar to qwerty, it might help retaining your muscle memory.

Indeed it is cool. I did it as an experiment and it really works, but I had a lot of problems with loose contacts. It is somewhat stable now, but if I move the board too much the problems come back. I have a tweezer in range all the time for quick fixes, and also had to replace the BOX jades and use 5 pin switches.

+1 for the Rommana. It is my daily driver when working from home. You can find my handwired build log in the referenced page.

I use two layers, that I activate holding the two home thumb keys, to place all symbols, nymbers and F keys. They are all positioned according to how frequent they are and how they appear together. I also have a lot of combos and macros (in another layer) that helps with recurrent commands. My keymap is extremely complex and optimized for my workflows, but you might take some inspiration from there.