This is indeed an ETB heavy build!
I do think that
Rite of Replication is a classic well worth considering. It's a clone for 4 that can be fetched with
Mystical Tutor, but late game the swing it generates can win games.
Imagine this on your own creatures:
Agent of Treachery,
Sun Titan,
Mulldrifter /
Cloudblazer, etc.
Cloud of Faeries is a card that I've come to seriously appreciate, and it's really cool with blink engines. Don't underestimate
Teferi, Hero of Dominaria's +1 each turn.
If you were looking to combo with Yorion,
Spark Double works.
If you went ahead and got a Mystic, I think that
Umezawa's Jitte is unparalleled. It absolutely destroys aggressive decks, and also accumulates a ton of value over the game, and can give you lethal if not dealt with.
My opinion about
Opposition is that it needs an absurd amount of creatures to do anything in commander, but once you get there it shuts down the game with nearly no contest. I slotted it into my
Kykar, Wind's Fury that poops out spirits all day every day, but I'm thinking it may sadly not be effective enough here.
I'm surprised about your inclusion of
Dowsing Dagger // Lost Vale. It's a pet card of mine, but it's also ridiculously hard to justify properly. I play it along with
Cloud of Faeries to untap for more mana, but you don't really have a way to abuse it. I'm thinking the
Cartographer's Hawk might be better in this deck.
Overall, I'm very confused by this list. It has a lot of janky blink cards and weenies that give you some value, but nothing that looks like a serious wincon, which makes me want to treat this like a more casual deck. Then I look at
Force of Will,
Force of Negation,
Fierce Guardianship,
Mana Drain, and they take me the complete opposite way.
So yeah, you got me. I'm not sure what else to recommend.