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AIUI seL4 runs on that chip.

It would likely not be a bad option.

Do you have more information? seL4 seems to run on raspberry pi 3/4, but I didn't find any mention of cortex-m class CPUs being supported.

Nevermind, I did not realize rp2040 is M0+ thus ARMv6.

ARMv7 is the minimum, and MMU is required. RISC-V is recommended.

This is a microcontroller (Cortex-M) not an application processor (Cortex-A).