approximation is to be expected due to divider and clock frequency limitations

I think you would have to turn wpa debugging on to see that line

You can technically update the bootloader and partition table via custom ota but it is risky and unsupported.

Check BOOTLOADER_VDDSDIO_BOOST menuconfig option. I dont have other suggestion.

If the ap the esp32 is connecting to is on a different channel, softap will also change to that channel. It should send channel switch announcement but maybe some devices don't automatically switch channels.

Yes, lower the fifo full threshold

It may work fine at wrong voltage for a while, but also may damage flash over time if it is above rated voltage. I cannot say whether it is connected to your problem but it is something to ask espressif about. Other than that I don't have any suggestion, sorry. I don't know what idf version you are ...

Ok I see how it is potentially confusing but the cache also works with flash encryption so there is no additional restriction.

No, where is that written?