Teensy++ alternatives

[Hak Foo]

I have a dream of building something along the lines of the GH-122... basically big enough that it will be a pain to try to wedge it into the number of I/O pins of a standard Pro Micro/Teensy/Elite-C controller.

The Teensy++ is expensive unless I want one of the Chinese knockoffs, and the supply long-term is questionable-- they're threatening they may not be able to supply much beyond 2021-2022, and if I have a stash of PCBs for future use, I don't want to be met with out-of-stock.

What else is a good choice for a giant matrix controller?  The Proton-C seems to be out-of-stock everywhere.  The CodeMercs Keywarrior-100 is interesting in that it's a purpose-built keyboard controller, but it's physically very large (especially in breakout board form) and may be less flexible than an all-software solution.

[Findecanor]

If you tweak the matrix, you could fit 122 keys with ceil(sqrt(122) * 2) = 23 GPIO pins, i.e. 10×13 or 11×12.
Easiest way to map it might be to have two physical columns per logical column.
Other options would be to strobe the columns with external shift registers requiring only 3 pins or an I/O expander requiring only two (I²C), leaving the rest of the GPIO pins for reading the rows.

The "Black Pill" boards come with various STM32F MCUs (same chip family as QMK Proton C) and 30 GPIO pins for around $5. I think there are several manufacturers (like the Pro Micro has).
There should be support for STM32F in QMK, Keyberon and ZMK.

[Hak Foo]

"122" was sort of a rough estimate-- it may well be a touch higher.  The one I'm using as a model is technically 131 keys right now.  And then I'd want I/O pins for LEDs.  And an encoder, since I'm being greedy.  So I'm looking at like 36 pins (main matrix being 24x7 or 25x6 without fighting with it, 3 LED lines, and 2 for the encoder)

[Findecanor]

If you'd double the rows going from 25×6 to 13×12, then the sum would be 30.

BTW, I have wondered sometimes if a rotary encoder could not be put into a keyboard matrix with two rows on the same column. Then it would have to be decoded as key codes...