I like the idea; I'd consider subbing out Quench for Censor and/or some Jwari Disruption  ^Flip. I also like Frantic Inventory , and more importantly with Mystic Sanctuary , Tragic Lesson is an absolute house. I also think that without looting effects, 4x Mission Briefing is a little much; honestly, card advantage is probably more important than recursion, as you're not looking to max out big spells so much as play a lot of them. If you mainboard some mass-bounce, it could be worth it to run a couple Briefing, but I think things like Behold the Multiverse , Chart a Course , etc could be more helpful overall.

I also think 3x Spell Pierce seems high - Negate or maybe Neutralize could be better. Curious Homunculus  ^Flip also benefits more with extra spells with colorless in their cost.

Also: since Anticipate doesn't trigger Chasm Skulker , it may be worth swapping for Deliberate or one of the other mentioned draw spells.

I also think with the full playset of Sanctuary, 4x Castle Vantress is too much as they don't add to island count; I think going down to just one or maybe two would be better, especially since by the time you'd be using them for value, you're generally either already winning or already losing.

I think Unsubstantiate is decent, but especially if you swap the other soft counters for hard ones, it may be worth changing out for Into the Roil to have a spell on four that clears the way for Skulker and adds to your draw count.

Finally, another card to consider (probably over Homunculus) is Jace, Vryn's Prodigy  ^Flip. Especially with lots of draw spells, you're likely to flip it early, and it both adds to draw count for Skulker and once flipped won't be bounced - though it would be better in a build with Frantic Inventory or other graveyard synergies to get it going ASAP. I think this swap is more of a tossup compared to the stuff suggested above, though.

I want mono U to work in pioneer, so I love this. Cheers!

Thanks so much for all of the suggestions! As you can tell from the rest of my decks on here, I have much more experience playing red and green aggro/midrange than blue, so these all seem like fantastic suggestions to get things to work together more smoothly. A lot of my choices were fairly simple choices based on reach and making simple replacements for a similar build to this I ran in modern (IE directly swapping Quench in in place of Mana Leak, Unsubstantiate for Remand, Anticipate for Serum Visions, etc.), so there's a lot I missed transitioning to the Pioneer meta, like how Anticipate doesn't trigger Skulker. I think all of your suggestions are excellent and I'll definitely move quite a few cards around based on them.