How fast can I cycle an AC SSR?
There is no harm in turning on or off a zero-crossing SSR at every zero crossing.

I did exactly that once in a system that had to control 24 heaters simultaneously.  The control algorithm produced a 0-255 value proportional to how much each heater was supposed to be driven.  The low levels used a Bresenham algorithm to decide each half-cycle whether each heater should be on for the next hal-cycle.  It worked very well.