I'm surprised that I've added about twice as many cards as you have (621) in the past 6 months. I too am working on a PhD, but to be fair, I've set aside time especially for Anki.

At the risk of other-optimizing, I might suggest adding a small number of quality cards daily. I used to add cards in bursts, but I've found adding a small number daily to be much easier. I also would recommend revising cards later, rather than starting over.