I really like this idea. Looks cheap, reliable, platform independent and easy to realise.
I think Confused/Garry Mitchell’s idea of DIY modulation is really cool. From what I understood the valve is controlled by a wax expanding/shrinking depending on whether power is On or Off, taking approximately 3 minutes in total to go from fully expanded to fully shrunken. The time it takes also depends on ambient temperature it seems. Mind you, it’s close to the valve, so it the ambient temperature there might range from 15C to 50C/85*C, depending on max supply temperature of your boiler, heating on or off, and heating duration. This might really be of influence. But you could imagine something like this.
Mode 0: Sonoff Off
Mode1: Sonoff alternating 45s On, 135s Off (alternating for as long as valve should be open)
Mode 2: Sonoff alternating 90s On, 90s Off (alternating for as long as valve should be open)
Mode 2: Sonoff alternating 135s On, 45s Off (alternating for as long as valve should be open)
Mode 4: Sonoff On
Not sure if it works. And not sure if the Sonoff and the Actuator will like the alternating treatment, but it could be a solution.. The lower your boiler supply temperature, the less you will likely need this modulation option.
Numbers above all based on 3 minutes to go from fully open to fully closed. You can check how long it really takes for your actuator by just turning it on without it being mounted, and just see how long it takes to fully expand/shrink. Once in the middle of the room (+/- 20C) and once while placing it on a tea towel or something on top of the radiator fully opened (>50C)