One thing worth remembering is that with any programmable keyboard you don’t need to actually use all the keys (and can even physically remove the switches if your board is hotswap).
You can see Ben did this himself to test out the layout with a modified version of his keyboard “the card” (looks like a different soldered board rather than hotswap: https://youtu.be/5RN_4PQ0j1A ) and again to demo his one button keyboard for the April fools video (with a one button version of the card—he must’ve had a ton of those PCBs laying around 😂).
So, if you want to try out his layout, you can do so without any worry by just taking any layout you think you’ll like, buying that, and flashing Ben’s layout to a subset of keys. A lot of the cost of a custom keyboard of this style is in the switches, keycaps, and microcontrollers (say 2/3 or 3/4 the cost depending), which if you make them all hotswap, you can keep reusing for many different layouts until you find one you want to stick with for a while.
All this is to say: there is a cost to get into the hobby, but experimenting after that point is pretty affordable!
HolyKeebs just announced their new keyboards with integrated trackpoints: https://holykeebs.com/pages/trackpoint . Something they do that is really cool is build them into the microcontroller itself, so if your board has the microcontroller in a convenient place, you can add the trackpoint straight to an existing device (already sell it for pimoroni trackballs, trackpoint in Q3). I have a Corne from him with the pimoroni, and I enjoy it, but the pimoroni is not a full mouse replacement for me.