Data is copied out of hardware fifo into ringbuffer when ringbuffer is full (threshold) or when frame ends (rx line idle).

Smarter to use rmt peripheral

The only things that run during flash write are IRAM interrupts. So the buffer will have to be big enough to hold all the data received during the flash write. Maybe you want to make sure the can interrupt is on a different core from the flash write. Using can hw filters could help. Or custom can is...

The best way is use an expander that has a built in led driver

If the binary is the same and you fully erased the flash then it could be a hardware rf problem. There was a report of a bad capacitor issue on this forum but I don't think there was any official statement about it.

It should be either 32 characters long OR NULL-terminated, but I haven't tried.

https://github.com/espressif/esp-idf/issues/3762

If you use an external can controller maybe you can use light sleep, otherwise probably not much.

80mhz is the max

I think esp32 is still barely supported on zephyr. Only uart,gpio,i2c are listed, no networking. I'm not sure exactly how that relates to successful build with binary issues but...