Even if you are using an RF certified module, your system (the final device) still will require testing. You will save money though, because it will require less testing than a full FCC / IC / CE certification of the underlying transmitter.

Only 5.57 US$ if you buy 5 at once...

SPIFFS does not have directory support, but it can be easily implemented using SPIFFS metadata feature. I have SPIFFS with full directory support implemented in my MicroPython for ESP32 . LittleFS with full directory support is also implemented. It can be configured to run on top of Wear Leveling dr...

Configure SPI not to use hw CS. Then control the CS line from software...

Yes, it should not be too difficult to adapt an existing SPI eeprom driver or write your own based on datasheet...

You are using the SPI Flash API for write/read, which is unrelated to the SPI Master API you have used to initialize the SPI bus.
You should write your own driver (or use an existing) to access the ST-25P80, which, BTW, is not a Flash chip, but SPI EEPROM.

Maybe you are using the wrong SPI mode. Try with mode 0.

Why not use ESP32-PICO-D4? It is only slightly larger (7x7mm), but you can save some space because of the components integrated on it.

ESP32 always executes code from SPI-Flash (some code is transfered to IRAM).

GPIO0 must be high on reset not to enter the bootloader mode, after booting you can use it as any GPIO.
Why is it held low and how is it driving SSR, via transistor or directly?
You should put a pull-up resistor on GPIO0 so that it is in high state during reset.