Here’s the latest on my build. I started down the poolcontroller route, but it started looking somewhat more complicated than what I needed. BUT I am taking your heating approach - set heater to highest temp and call for heat based on the sensors attached to the water line. Pretty ingenious wo having to deal w RS485 stuff. So my build will ultimately control filter pump, 2 valves for pool/spa feature, heater, pool and spa lights, and spa blower. 
