How to reduce the leakage current of a solid state relay?
The 'leakage' is no doubt coming from the internal snubber network. Perhaps they increased the capacitor value from when the datasheet was written. 

The load resistance is R = 240^2/2000 = 29\$\Omega\$, so with 25mA leakage you'll have a power in the heater of 18mW (I^2R), which is not a problem. The '13W' you calculated is actually 13VA, almost all leading in phase and does not represent power either in the relay or the load. 

So, I don't think you have a real problem. Your cycle time is a bit short- you may run into beating issues with the mains, but that's another issue. 

Anyway, to answer your question, the easiest way to avoid the snubber would be to make your own SSR with a opto-triac and a snubberless triac (aka alternistor). But it's not necessary.