What esp-idf version is this? Can you try updating to the master branch to see if this is still an issue?

It smells like somehow WiFi is radiating into the EN line, which makes the chip reset; that's most likely if there's an inadequate or no pullup connected there, but it's hard to check that without a schematic. As I said, adding a resistor (anywhere from 30K to 1K will probably work) between EN and 3...

FWIW, there is a lightweight CPIO extraction library in the ROMs of all ESPs with USB-OTG support: API is here; it's used in DFU firmware download mode (an ESP DFU firmware binary actually is a CPIO archive). If there's more interest, I can see if we can also publish the source to it.

Those are the defaults for SPI2, iirc; not for the main SPI flash. I think you can still fix this by burning the efuses that tell the chip which pins to use for the main flash, look here for more info. Generally, it's advised when designing a PCB to look at the hardware design guidelines and follow ...

Wondering if there is something wrong with your board... my guess would be a wonky resistor on the EN line. If you have one, you can try adding an 10K between the EN and 3V3 pin, see if that helps.

How did you pick those GPIOs? They're certainly not the default ones for the main flash. As a reference, look at figure 5 of the c3-wroom-02 datasheet.

I think you were on the right path with the separate task; you really shouldn't put intense and long-running things in a timer callback (as it stops other timer events from happening as long as the callback is running). You probably want to take a look at vTaskDelayUntil to solve your other problem.

Phew, it gets a lot more complicated then... Can I ask in what situation you're runnning into a timer stack overflow? If it's in your code, it may be fixed in some other way.

As a last resort - while it is connecting, can you hold the 'boot' and press 'en' shortly?

This piqued my interest. I couldn't find anywhere in the datasheet - or in the technical reference manual, or hardware integration guide - any mention of GPIO16, GPIO17 recommending not being connected for flashing firmware. [...] What are the specific details around this constraint? TIA! It depend...