Hey so I am not nearly as experienced as some people on here but from everything I know drooling is normal with suggies.
I know at least that my two girls will sometimes drool a little when eating Mealies because they get excited.
Also the diet you are feeding them sounds very good as far as I'm aware. I personally dont feed TPG though I feed OHPW but it sounds like a good diet.
Overall in my opinion and from my experience drooling when having treats sometimes is normal so I wouldnt worry.