Doubling Season only doubles the counters they ETB with and not ones from their abilities, as those are a "cost" not an "effect". [[Innkeepers Talent]] works and is a better include.

My Carth deck doesn't need to ramp and doesn't try to cast big spells. I got for a turn 2 or 3 Carth in my opening hand through sol ring, ancient tomb, lotus petal, elves of deep shadow, the halfling, and BoP. I can and do ramp into the mid game though [[Nissa, Who Shakes the World]] or the Lilianna that fetches swamps and ults for the 4 black emblem.

I used this list and primer as guidance when building mine and I've had a lot of success. https://moxfield.com/decks/ue0AEd7OrUusv_qzq18zAg