The main problem I see with Keening Stone is the mana cost. Even if I don't activate on the first turn 5 per activation is a little steep and on most turns that would take up most of my mana. Comparatively, Sewer Nemesis and Consuming Aberration provide additional mill via abilities, can usually block creatures of any power as soon as they hit and are free to activate after the initial investment. Also I can untap them with Umbral Mantle or Thousand-Year Elixir

This is already a highly tuned deck. Not sure i can suggest some improvements.

Maybe you can try Havengul Lich for reanimation and Paradox Engine for untap shenanigans.

Also i think Deranged Assistant is an odd one.

My Decepticon has been shelved. The pieces currently sit in my Mind. Please have a look and suggest if you could.

Deranged Assistant, in addition to being a rare blue mana dork. Is mainly in there for a combo I found for Necrotic Ooze when it shares a graveyard with Pili-Pala. I can mill my own library at mana speed and briefly access every activated ability, including Tree of Perdition and Eater of the Dead.

Paradox Engine doesn't really shine in a deck with such a heavy focus on permanents over instants and sorceries but Havengul Lich would certainly be useful. Especially in conjunction with Teferi, Mage of Zhalfir. Unfortunately, I'm not sure if there's a card besides Teferi that I could swap for it.