intelligent full cycle control needed, 5 out of 6, 2 out of three, 1 out of three, 9 out of 10 ( full cycles ) depending on temp target, a max input current limit needed ( CT or calculated based on Vac in and % of full cycles out ) however unless your switching is very precise about the zero crossing - you may well get a small net average DC component in the neutral ( & phase ) many countries limit thjis to 5mA DC max for continuous operation ( for domestic earth stake corrosion reasons ) - so there is this to be considered.
Also there are flicker implications as FvM mentions
An SSR with a suitable inline choke would at least soften the current waveshape, turn on at zero voltage, off at zero current ...