Cool that you're picking up the idea. I've had my list ready since last October, just never got to to it to action since Leyline and then Control piles took over & pushed both Phoenix and Shadow out of the meta until Breech came and went. Perhaps now is the time to test this at last?
Definitely agree with Delver over Peezy. And I personally like Entomb over Buried Alive + Ritual here; it's more elegant and opens cool tutoring options (Bloodghast, Shenanigans, Cling to Dust). I also think the Buried Alive plan is too risky in a world where everybody maindecks 5-6 Forces. Royal Scions are a strong consideration as a one-of; Tarnished Citadel another cute option once you're in 3 colors. Snuff Out is a must. And no Peezy --> no therapies.
But you don't have to brew this from scratch either, have you seen this article?
https://boltbird.com/p/grixis-arclight-shadow-a-primer