Nice, it does look a fun project though!

From scratch? Or a kit?

I’ve never tried a wireless keyboard (despiteowningmore than 50), still waiting on my Pearl three years after I ordered it lol, that would have been my first.

All my splits are cabled, so it’s no issue for me.

Another option is home row layer keys, duplicating the layer options in pairs so a and h, s and j, and so on so you have the same layer from each side.

Smaller boards I use home row mods (control, super, and so on), and that works extremely well for me.

ZMK also has a layer tap key that would suit for this, normal key press of your choosing and layer activation while holding: https://zmk.dev/docs/keymaps/behaviors

I have a large selection, but I rarely use them for anything other than media keys and teams call functions. Other stuff like windows management in sway are all done on higher layers on my (40%) keyboard.

My preference is for something with at least one knob as knobs are my preference for things like scrolling, volume control, and zoom.

If you can find latching switches, these are great for triggering layers so you can get more out of your pad. You can obviously do latching layer changes without such switches in QMK, but you really need to trigger leds to help you remember which layer you on otherwise.

Pad I use the most is a CapsUnlocked CU7 that I got just before they imploded as I love the weight of the knob.

Do you mean keys thst you hold down with your thumb while your fingers are on the home row or something else? For the former, I use split space keyboards and map the left one to enter on press, layer on hold, the right one to space on press, and a different layer on hold. That gives me more keys that you would get on a 70+ keyboard. I find this considerably easier than trying to use pinkies on shift and symbols on what is my top row, qwerty row.

Typically, I map tab and the traditional enter key to tab on press, another layer, and backspace with the same layer as tab. This layer I use for window management in sway for home row and anything else I am missing that I really want.