Upon reading rfnovo’s generic thermostat suggestion above, I got excited to try it. I increased the pool’s physical heater setpoint to 35°C, and in initial testing Home Assistant control seems to work perfectly well.
Note: If someone turns on the heater directly from the pool fob or control panel while the Home Assistant thermostat is “off”, the pool would get heated to its own (excessive) set point temperature, beyond Home Assistant’s. To protect against that situation, I created an automation to watch the pool heater and automatically turn on the Home Assistant thermostat so that HA can resume control.