Since I got one of those thermostat with the choice of dimming and pulse-proportional I wouldn't go back to on-off. There are some relays that implement this directly and you control with PWM (like pc fans). The alternative is writing your own pulsing algorithm which might not be easy. I can't find digital dimmers for more than 12v, some suggest putting a servo on an analog one but I don't like the idea.

Any way in your schema to control the power absorption too? That would be an interesting stat to log and one more parameter to check for failures.