Raspberry Pi is a difficult target. It is not Open Hardware, and there is a bunch of critical closed-source system software. The Broadcom SOCs are not fully documented.

All true, but there are a few documents floating around like: https://datasheets.raspberrypi.com/bcm2711/bcm2711-peripherals.pdf

Support for the RPi zero with the BCM2708 was started and was partially functional, but was never finished:

 * 
https://bitbucket.org/patacongo/obsoleted/src/master/nuttx/arch/arm/src/bcm2708/
 * https://bitbucket.org/patacongo/obsoleted/src/master/nuttx/configs/pizero/

The RP2040 however is a complete different thing. There is no Broadcom involvement, and the chip seems to be fully documented, and there is a NuttX port.
Beaglebone Black is another supported SBC option.  Also the older pcDuino SBC.

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