From memory, the Tx fifo starts sending as soon as you write a byte to it. The idea of the FIFO is that you can write a fair amount of data into it in one go, faster than the data is sent; if you're doing the sending interrupt-based, it means you don't have to fire an interrupt every byte.

Btw, just to confirm, this is memory corruption: your code somehow writes to a location it should not, corrupting the meta-information of the allocator.

On which chip? The ESP32 doesn't have this, later chips (S2/S3, C series) sometimes do.

Yes, it probably is configured by default to JTAG. Use gpio_reset() to reconfigure it to a plain old GPIO.

What specific GPIOs is that connected to? It looks like you're connecting to GPIOs that also occupy the flash pins (or something else is wrong with those GPIOs - shorted or something), which would stop the ESP from being able to communicate with its flash.

No, it does not support external SPIRAM (except as a generic SPI peripheral, where you would need to manually write bytes to and read bytes from it).

You might need to use menuconfig to set the standard log console to CDC/ACM.

Quick hack that probably does what you want: change to

You can do very little in an interrupt handler, and specifically doing anything that block the calling task will crash the ESP32 (as an interrupt handler is not a task). The proper way would be to use FreeRTOS tasks and semaphores (or other inter-task things that FreeRTOS provides): the task blocks ...

DFU 'run' mode needs support in the app. If you use esp-idf, using menuconfig to set the logging console to CDC/ACM should enable that.