IT WORKS
I got a first working version, using an ATmega1284P running on MightyCore 2.2.2 (as soon as I began developement, they switched from Optiboot to Urboot in version 3.0.0, whicht does not include the copy_flash_pages function anymore, but I'm working on a work around there).
Only thing I could not solve is to do a CRC check of the image after download...
You can try it out here: https://github.com/eiten/MySensors/tree/FOTAInternalFlashTest
It's not beautiful yet with much Serial.prints in it, but maybe it helps you if you find errors.
@OldSurferDude well, the ESP8266 is limited to 4 TCP clients in arduino IDE. This can't be really increased. I did som experiments and you can set it up to 15, but after the 5th client on my web server, I got a freeze.
Maybe you got something wrong. #define MY_GATEWAY_MAX_CLIENTS 2 defines how many controllers (eg Home Assistant) can connect to the gateway, not how many sensors/MySensors devices.
And yes, you can have multiple TCP gateways in HomeAssistant. I got an NRF24, an RFM95 long range and a RFM95 short range gateway (all based on ESP32) on the same Home Assistant.
Regards, Edi
Thanks, I'll keep this in mind if I decide to debug it. I realized that I am also tired of patching the gateway through to the doctor container inside an LXC so now I am looking at creating an esp gateway.
Just need to check if I can find a pcb on here.
Thanks again!