[Lain]

I would actually recommend an arduino.

you can program them to do anything at a low level, and I know for a fact that people have programmed them to do everything from DIY keyboard inputs to custom HOTAS setups.

however if you want to take it a bit further... ever consider building a circuit with something like a bus switch or something similar?

you could switch inputs by setting a pin from low to high with a mechanical switch thus activating your digital input/output.

I've been working on an AV switch using this principle with potential to be more than just AV, and my early tests have got a dual input audio switch working so far for less than $5 in parts.

using this principle what I would do would be to get a simple programmable Microchip, and use the bus switches to change information to pins using a mechanical switch, and power the whole thing off usb with the microchip sending the information through a USB bus chip.

in total, maybe $15 in parts, at most, including a case if you need one, a power regulator circuit, wires, and the mechanical switches, even LED's if you want them.

Edit: re-reading your post I see I missed the original intent.

honestly what your best bet is probably to look at how the macro pad is wired and see if you can find the signal leads, it shouldn't be that much different though, a key switch

failing that, it seems a lot of DIY mechanical switch boards use a teensy for programming.

you might be better off building a macropad utilizing a teensy or an arduino from scratch over modifying an existing one, I know it can accept inputs and outputs so you could in theory program it to record sequences fed through it, but you'd have to put it between your keyboard and your computer so it can capture the USB signals without dedicated software.

Edit: useful link!: https://www.makeuseof.com/tag/make-custo...s-arduino/

either way you look at it, it's going to be a hassle though, and to use the existing one would require figuring out the input and output of the macro switches and the mechanical key switches, once you do that though it shouldn't be that much different regarding input/output variables.
the macropad is just recording on/off states caused by keys pressed down and from that button activating a saved keypress macro.

have I done this? no, but I've done research into converting cheap keyboards into mechanical ones so I can tell you a bit about the conversion process.

you'll also need to be aware of the resistance differences between the two types of keys, it's small but it builds up depending on how far you are from the actual brains of the device.

Using an arduino isn't optimal in the slightest lol. The only thing arduino has going for them over any other MCU on the market is their own language/HAL, IDE, and forums. I've got a couple STM32 boards lying around at a fraction of the size of an arduino uno that have clock speeds 8x faster than the atmega328p, only downside is that you need to use C/asm to program them, and not some derivative 'simplified' language (although they have a very extensively documented HAL.)

I'm using a Teensy already, mainly beause keyboard input support is already  pretty decent on the atmega32u4, or at least miles better than the 328p which generally relies on ps/2 or other host hardware to do anything related to non-serialized input.

The only issue I have right now is that the thing looks like frankenstein because it's still all connected to a breadboard for prototyping, and I want to make it actually presentable so I need to start soldering stuff and adding some 'real' non tactile-push-buttons, hence why I'm debating either actual mech switches or the Trellis/silicon pad.
I can take a few pics of it when I get home. It's definitely not something I can just bring to class or something without the whole building thinking I have a bomb.

I'm starting to think Trellis would be better because of all the cool RGB shit, not to mention it'll look a lot more unique than most people's macro pads, just don't know what the resistance will feel like, or exactly how it's wired up and built.

There's definitely more tutorials and stuff on the subject of soldering mech switches, but generally those guides use Gaterons or Kailhs instead of authentic cherryMXs which AFAIK need their own breakout board to do anything useful like anti-ghosting, but I could be wrong.