Raspberry Pi 500+ boasts a high-quality mechanical keyboard with removable keycaps and individually addressable RGB LEDs, an internal M.2 socket pre-fitted with a 256GB Raspberry Pi SSD, and 16GB of RAM.
Would be pretty darn easy to cut the slot in the case. And print a plastic cover that clips in.
You have to do a bunch of SMT soldering to add the circuitry, doesn't seem worth the hassle, especially since I wouldn't use the 500's built-in keyboard anyway. I wish it had included a pointing device like every laptop has. Using a separate mouse means you also need a desk to put it on, which is constraining. So I think I'd get a pi 5 and external keyboard if I wanted to stay with Pi stuff. There isn't that much attraction though.