Hunts is a mess but that solution is too complicated.

Make S Rank spawns more common (reduce the reward for each kill), between all the zones there should be one window open every 2 hours or so.

Make A Ranks the Weekly Bill and B Ranks a Daily Bill.

This will get people out there but not actually zerging. You can tie guaranteed Cracked Clusters to the A Rank Bill and/or have a % from the B Rank Bills.