Hi, I can speak as a TT card collector myself (currently only missing the one in Doma Castle). The drop rate is fine, it adds to its collectable value and it works as an excuse for SE to make people run content over and over, filling the seats in DF. Bear in mind that some cards will eventually drop from NPCs being introduced throught the expansion. As for cards like Shinryu, the encounters that now are "hard", will become easier overtime as people overgear it. There's no rush to complete your collection, just enjoy the journey.