The Pico comes with 2MB of flash rather than 8MB, but same idea, more or less. There are some 3rd party boards with 8MB. I suppose you could connect up an SD card via SPI for near-unlimited storage.

This might be of interest:

https://blog.adafruit.com/2022/03/10/linux-written-in-python...

It's a clickbait title, it's actually more of a busybox-like thing giving a linux-like shell from what I can tell. I haven't looked into it.

FUZIX, which I think is based on Doug Braun's even earlier UZI (Unix Z80 Implementation), might also be of interest:

https://www.raspberrypi.com/news/how-to-get-started-with-fuz...