Inform the cops about the invasions and reinforce that opening. If they break in past the fixed version, they're after something for certain. Maybe you can consult an attourney to see if there's any way to have the cops search for drugs without you getting charged for the possession? I've always wondered what happens if you live in a house that had druggies in it previously and they stashed a bunch and then you have the house searched or it gets searched. If it's like cars, it's possession.
Anyway, it'd might be worth it to stake out your property from a friend's van parked across the street. They have to know when you're home and not, so they have to be watching from somewhere. Make note of the cars within sight of your house when you get ready to leave in the morning and when you get home at night. Come home early every now and then, and randomly, to throw off their schedule. Search every inch of the house at some point!! They're after something, the more I think about it.
Finally, be ready to protect yourself after you fisx that door. They might be willing to become violent after realizing you're onto them. Also, you might have the police look into whether someone previously an occupant might have just gotten out of jail. That might help in some way to track them down.
Good luck and be safe.