Best strat seems to be doing a small number of invasions daily rather than 20+ invasions in one day. Based on my experience and the some of the reported experiences of others, it seems like drops rates are higher when done daily than ground out in one day. I’d argue do 3 invasions anytime you feel like pursuing it and then capping it after that if you don’t get it. Priority contracts take 3 runs daily in contested zones so I wouldn’t be surprised if bungie had some similar drop rate system for crystals