You are welcome. I did check the ruling on
Desecrated Tomb and it only creates one bat at a time even if many creatures would leave the graveyard: "7/13/2018: You create one Bat token each time
Desecrated Tomb’s ability triggers, no matter how many cards left your graveyard."
Just brainstorming, but good single activation graveyard cards could be
Reassembling Skeleton and
Teshar, Ancestor's Apostle.
Liliana, Untouched by Death would need to be built around with zombies.
Blood Operative is a bit expensive at 3 life, but would not need mana to leave the graveyard and might work together with Kraul, Swarm.
Narcomoeba, would be great, but that's in blue.
I could even see a use for
Lich's Mastery with the setup you have.
Exile a creature, gain life, create bats.