I think Niantic releases more “Catch 5 big-type pokemon” field quest now.

That would be nice. I've only gotten 1 so far (although it was a 100%)!

Yes, it's been an easy quest that's really hard to find. I comb through several tens of stops to try and find one every day. I've gotten 4 so far.

Im on the opposite side. In first week i found no less than 2 per day( it was between 2-5) but now i didnt find any last days.

How easy it is depends on circumstances. My second one was accompanied by bugs vanishing nearly completely from my area. Took me three days to catch five of them, and one of these was Shedinja from the quest reward.

I hope so. I have been trashing almost every quest I get, and spinning lots of stops, and still no nincada quest. I would hate to have the whole month go by without one.