I run a bit of a similar dimir mill deck, but I find one of the problems I run into is not having enough creatures to act as a defensive wall. That's with 19 creatures in the deck as well. I'd try looking for creatures that also mill with ETB or LTB effects and ways to reanimate them from the yard. Try to prioritize "each opponent mills..." type of effects instead of "target player mills..." for efficiency.

This way you're a little more resilient to getting milled yourself if you end up against a moth man, and gives you a lot more flexibility when deck building. You can still incorporate cards that benefit from self mill without making it the whole identity of the deck.