quick (silly) update. I was able to get pico-8 upgraded on my CubeXX running Knulli (Gladiator II) after all. I was dumb, I forgot that Knulli has an awesome feature of just copying the raspberry pi .zip file for pico-8 and do a system install of it. and then "Robert's your mother's brother", it all works. Umbra is running on it. I hope that this hopes some other poor soul in the future!
PS, AI is not your friend in this case, but Discord is! :)