Do you have some tips on how to create a PCB, which controller etc to use? Since you've made your own board, I'd be interested in getting some advice on this front, maybe some common pitfalls etc? My board has lots of keys, so I assume there's some limitation on what I can pick there. I should be ok with soldering/lubing/etc when I get there.
So, I only JUST graduated to using any sort of PCB at all. Until the one I finished yesterday, all my customs had been hand-wired, manually soldering the diodes and partially insulated wires to the switches and a standard Raspberry Pi Pico variant with USB-C (they're available all over Amazon, AE, etc.). I watched Youtube videos and read tutorials and managed to cobble together the matrix part into a PCB, which I had made at JLCPCB (under $30 for the minimum order quantity of 5, but I had literally no components, just holes and traces), and I simply left 23 though-holes to solder to my Microcontroller board, same as if it were a hand-wire. There are open source reference boards out there, like the Boston or the GH60, that you could download to see a more complete iteration. Some people split the difference and still use a separate microcontroller dev board like a Pro Micro or Pi Pico, but solder it properly to the board or use a socket. For your initial layout, make sure to use one with a bare minimum of 22 accessible GPIO pins, and more would be MUCH better. You absolutely have to be at least [2 * sqrt(number of keys), rounded up], but that makes for a weird bit of wiring since boards are rarely that close to square. The 26 on a Pi Pico would let you make a fairly reasonable matrix, though I luxuriously used up 23 to make an 85-key board.
I am fairly handy and mentally flexible, but my educational background is almost as far from Electrical or Mechanical Engineering as you can get, so this journey has been one learning process after another. If you're farther along, you could prbably just look at a KiCAD schematic and layout, and modify it accordingly for fabrication. You might even consider glancing at the mCAD files for the Boston to see if any of its parts could be reused or manipulated to avoid starting from scratch. the 3D printed version seems to have seams in many useful places.
Now, if you are starting from nothing... no worries. Bigger is not actually harder, just... bigger. If you want to consider the "minimum viable design" you can make a board with two 1.5mm aluminum plates and a bunch of 10 or 15mm standoffs. I would suggest making your final layout in KLE, then taking it to swill's plate generator and getting some DXF files. You can open them in most 2D CAD software, including LibreCAD (free!) or even Inkscape, though the line-widths always throw me, so I just do that the last second for my home laser-cutting needs. Once you have it open, I'd add a few interior screwholes for your big boy (flex is one thing, but this is NOT a 60%) as well as some space and mounting holes for your MCU. Then use xometry or some other vendor to fab a switch plate and a bottom plate.
Hand wiring is extremely straightforward and is a good way to really understand how a keyboard matrix must be laid out, once you do the "paperwork". Joe Scotto has some really nice and easy-to-follow videos, though he's making showpieces with his ginormous webs of copper rebar (lol), and there is a wealth of information here on GH. An aluminum plate is relatively cheap to have cut and is more than strong enough at 1.5mm to accommodate simple plate-mount stabilizers. Programming in QMK/Vial or the new POG tool for KMK (I just tried this and LOVED it) shouldn't be too hard. I'm biased, but making your own keyboards is one of the biggest bangs for your buck (and time) in the hobby. The way they function pretty much exactly like a professionally made product is just deeply satisfying, as is knowing that you have the exact keyboard you wanted.
Now all that said, I'll circle back and continue harping on keycaps. Research available sets and decide what your needs and preferences are and what your budget is. Getting away from the keys used in standard layouts (or a few more popular variations) makes kitting hard. It just does; not impossible but hard. If it's your original layout, you will likely be looking at XDA or DSA, since you can supplement whatever set you get with blanks, and your dreams of a 2.75u Enter key can be a reality. Having stuff on the wrong rows just feels bad, man. :-)
Good luck if you decide to go in this direction!
