Earlier quoted context omitted.
If you're not designing your own microcontroller, you might want to check out the Proton-C. It has 248kB of usable flash memory, which could be enough to store a stenography-optimized word list in addition to the firmware. It was made to be flashed with the qmk keyboard firmware, but I don't think there's any requirement that you have to use qmk. https://qmk.fm/proton-c/
I used an STM32F103 for my last keyboard, and you can buy four for the price of a Proton C. I am still not sure why they chose the F3 series for the Proton C, as it doesn't seem to be a chip that's commonly used in the maker community. I am somewhat surprised they didn't bite the bullet and just port QMK to the nRF52 (since everyone wants bluetooth out of the box), actually. The F103 is also interesting in that it is…
I was initially thinking about something that is already bluetooth enabled like ESP32 or CC2640R2 (I have development boards for both of these) but the design suddenly gets exponentially more complex if you want to get it right. Also, neither has a package that I can easily solder. I know it is just aesthetics, but I really like a clean board.