Yesssss. Any suggestions ?

My idea was to use a low frequency PWM, probably about 100Hz. That would also allow the thermostats to operate safely.
In order to regulate the panel voltage the feedback has to be the opposite way round to a normal switched-mode PWM, so you need an old-fashioned voltage-mode controller which gives access to both inputs of the error amplifier - such as a SG3525.

 

If you use PWM, then the arc will extinguish during the off period so you may get away with it.
But, as pointed out earlier, in winter, the solar panels will not produce much power.
For example,at the moment, with heavy cloud, my 10KW solar panels are producing 800W. It may go up during the day, but that is an indication.