It occurs to me that if one wants control of the heater set point, it could be accomplished by ceding full control of the heater to Home Assistant.
One would start by setting the heater’s internal setpoint to some exceedingly high (or max) value, higher than any temperature you’d ever want the pool to actually achieve. Then in Home Assistant:
establish a helper to store your true desired setpoint temperature value (lower than the heater’s internal setpoint from above)create an automation to watch the water temp to turn the heater on when it drops a couple degrees below the helper value, and turns it off again when the water temp reaches the helper value- replace the heater on/off button in the dashboard with
one that simply toggles the automation on&off (though you might want another dashboard entity to indicate the heater itself is actually on)a generic thermostat
Haven’t tried it myself, but I think it should work.