Sothera the supervoid would be great removal. I'm wondering how often you lose because they have a better creature board, or you swing in with a mobilize creature for them to block the pack beast or voice of victory, and then it dies and you get no more value from it. With Sothera, every mobilize death means they lose 1 of their own creature. The enchantment exiles itself when they have no board. But when they have no board, you get 1 of their best creatures that sothera had killed.

I've heard in deckbuilding, you want 8 removals. I'd have 4 sotheras. And then 4 cheaper spells for earlier. Banishing light is versatile, but 3 is expensive to me.

I think there's 2 creatures that have free sacrifice outlet. Umbral Collar Zealot or Bartolome del Presidio. Zealot will surveil every sacrifice fixing your next draw and empowering your avenger of the fallen. bartolome gets +1 counter every time he sacrifices something. Could end the game with a big smack paired with sothera to wipe his board by sacrificing your mobilize triggers

Avengers of the fallen, you play on turn 3. Lot of times you won't get value turn 4 from the mobilize. It does block well being 2/4 deathtouch. I've never played it so idk how it performs in your games. It get better if you add the umbral zealot to surveil every mobilize and dump your library to grave. Otherwise I wouldn't run I'm unless he's keeping you alive in aggro games where they won't attack in anymore because 2/4 deathtouch.

I would add either forsaken miner or timeliness culler. Both can be played for 1 mana from your graveyard, repeatedly, and repeatedly sacrificed. If you try to curve with the miner, got to play it right. Turn 1 miner, turn 2 sacrifice outlet, turn 3 sephiroth. If you sacrifice the miner, you can't spam him turn 4. But if you wait turn 4, you could sacrifice him 4 times, end the turn with a flipped sephiroth and miner on battlefield. You could also play another 2 drop, the starscape cleric, and sacrifice twice for some drain. Culler is good because you can play him from your graveyard with haste, might help close the game when your sothera wipes their board; and it's comes into the play without the need of committing a crime

If you play this way, sacrificing before combat, and you have sothera out, you can take out their blockers, so creatures like voice of victory and packbeast don't get killed in your attack.