I think the I2C slave implements some kind of clock stretching now, so there's at least half a ✓ more

No, the ESP32 only has two SPI buses free, and they do not support DDR. You may be able to fake something with e.g. the parallel mode of the I2S peripheral, but that depends on your use case; if you can tell a bit more about what you need this for, we can probably help you better.

Erm... it's exactly the same as the external flash, the chip die is just located inside the SOC and it's connected to some slightly different pads (for physical reasons). Anything specific you want to know?

Alternatively, if you don't care about exact I2S frequencies, you can possibly also run that from the BPLL and free up the APLL for Ethernet.

Gotcha. First of all, you can still do the core1=wifi/bt, core2=peripherals split even with FreeRTOS. The trick is just to make sure WiFi/BT runs on one core (you can select this in menuconfig, I think it defaults to core 0) while you start the tasks to initialize / handle the peripherals on the oth...

In particular, show down your adapter. " adapter speed: 20000 kHz" means it's running at 20MHz now.

The only safe way is to have physical access. The issue is that the partition table is something that only has one copy in flash, and if something happens while you're re-writing it (e.g. power loss), your device is bricked. If you still want to do it, you can possibly use the raw flash functions to...

The standard way of doing things would be to always compile A, B and C, but to use e.g. preprocessor macros in your main program to not call any functions in them unless they're selected.

Is your terminal using a cr/lf combo to end lines? I think that's what the AT firmware expects; if this is set wrong, it'll think you're still busy entering the first line.

If anything, you're setting SPI_TRANS_USE_TXDATA, which means that the SPI peripheral will get the tx data from the tx_data member, but then you're feeding it a buffer through the tx_buffer member instead. Remove the SPI_TRANS_USE_TXDATA flag and you should be at least a bit closer to working code.