If you are not on a budget, try running some big staples, such as Nirkana Revenant, Liliana of the Dark Realms and others.

Take a look at my mono- EDH deck for more inspiration! +1 from me!

What about a Rune-Scarred Demon? That'll be at least 2 tutors on a flying 6/6 body. Check out my mono-black ETB creature extravaganza: Chainer's Graveyard Circus!

I would suggest swapping out Phyrexian Altar for Ashnod's Altar, readding Cauldron of Souls and a Rune-Scarred Demon. With Mikaeus, Ashnod's and Cauldron on the field. Play Rune-Scarred, tutor, give it persist with Cauldron, sac Rune-Scarred to Ashnod's for 2 and return it because of persist, Mikaeus undying trigger removes the -1/-1 counter, repeat for infinite tutors and infinite colorless mana.

It would lose the persist after the first time it was sacrificed, so it cannot go infinite.

Oh right, I forgot you need Dross Scorpion to untap the Cauldron, thanks for the nudge Ayle

The same infinite combo can be much more easily achieved with any undying or persist creature, Ashnod's Altar, and Nim Deathmantle. You don't have infinite mana but infinite death and enter the battlefield effects. Same can be achieved with things like Grave Titan or Rakshasa Gravecaller that bring other creatures in with them. Grave Titan hits this combo and goes infinite zombies and infinite colorless mana.