You just 100% contradicted yourself in one post. You say Cling to Dust should be very good for this archetype (a card that not only costs 4 mana but needs you to have a graveyard to exile as well) and then said 4 mana spells are pretty ambitious. I'm not really sure how to take your post. The first use of Cling is fine but the Escape is what makes the card considerable at all, and that is the place where it gets ambitious. In blue-based stew decks this card is good at padding life total and drawing cards, alongside cantrips to feed the lands to make it work. In Pox you're actively sacrificing your own lands and living off the top of your deck. I would play other options before Cling to Dust.
If you're on the Karn plan I think you need to have Lattice, it's just too good to leave out. You will need Dark Ritual or some other mana acceleration, maybe Ancient Tomb.
In other news: boys, we got 'em.
https://i.imgur.com/phFDdDh.jpg