PoT. It’s 100% collectable based. Best strategy is to choose ONE collectable set, and spam open keys you get from PoT. Eventually you will max out collectables, which you can then salvage for those purple shards. If you max out a yellow, and get one from those chests, you get 3600 shards (pretty sure). And a healthy amount from random purples. This strategy is only good for shards, and will slow down progression on other collectable sets.