@marc-r I'm not so deep in the MQTT/Tasmota Topic. So not sure what you can do with Tasmota, here in this use case we want to put a serial Input somewhere. When Tasmota can do that, you should be okay with that. I was thinking about a little Programm on ESP putting the serial e.g. on a Webpage. This is probably a few minutes programming Job. I think i prefare this, because my Pool also will be controlled by an ESP. If a Server is in the middle is one more thing that can fail.
But anyway i can wait if the other guys here find the "LAN Protocol" Solution. If i can use that from another ESP, i will probably make like this.
Well i just thought i can connect the 803W serial connect directly my control ESP ... this is also a good failsafe alternative.