There’s definitely custom ROMs; I run one on my current phone. You should too, if your model makes it possible - they tend to be OSS Android forks and can do whatever the stock one can, but better. (DivestOS being my personal choice, for the Google-freeness)

I suppose I could have cut out the SoC and replaced it with the same SoC but not locked already. I didn’t think of that, lol! Maybe I still could - it’s still relatively new, but selling the thing feels like letting a great evil back into the world. I have no idea how hard the particular one is to pull apart in a controlled manner.

Using a different chip would be pretty hard. You said microcontroller, but a phone is closer in function to a desktop PC than a dishwasher. There’s high-bandwidth things going on and you’re going to need a lot of bespoke circuitry and software to kludge it. Forget about the end product having the same form factor, too.