Need help to design efficient zero crossing CURRENT detector : clamp diode solution
I select STTH20R04G-TR diode because they can handle more than 16 Arms


This is an 'ultrafast recovery' diode which has a 'softer' V/A curve than most rectifier diodes. This will waste more power at high current, but still drop about the same voltage as other diodes at low current. The low current behavior is important because the TRIAC holding current could be quite low (the datasheet says 50mA maximum, but does not specify a minimum). 

Whether the diodes will need 'extra' heat-sinking depends on the current draw. Letting them run hot reduces heat-sinking requirements because the voltage drop decreases, but may adversely affect the current detection threshold (as well as not being good for adjacent parts on the PCB).  

You have calculated the optocoupler input current based on rectifier voltage drop at high current. At low current (eg. 50mA) it may be too low to operate the optocoupler, and so may falsely detect that the TRIAC has turned off when it hasn't. At 1mA the typical optocoupler LED voltage is ~1.025V, so assuming 0.6V per diode the required value for R10 is ((0.6*2)-1.025)/1mA = 175&ohm;. At 3.4V the LED current will probably be around 10~15mA. 

The snubber could be a problem because it bypasses the TRIAC and injects current into the diodes and optocoupler. At mains frequency it might be OK since close to Triac turn-off the voltage is low anyway, but at higher frequencies the capacitor becomes a short circuit so noise will be amplified. This may cause glitches at the optocoupler output. You could try moving the snubber to the other side of the diodes so the optocoupler only responds to actual TRIAC current.