DedWards

Brave is a solid sideboard option. Not sure if there's space in the main deck for it though.
Angel is a great option. Definitely worth testing.

Thanks for the suggestions ??

It was those suggestions that led me to Oversoul of Dusk as they're all part of the same cycle of cards. I even considered some of the Lieges from that block.

Pauper is a format where you're only allowed to use cards that where printed as a common in at least one of its printings.

Manamorphose was a common in Shadowmoor
Flame Jab was a common in Eventide
Highland Lake was a common in Amonkhet

Problem with Curiosity is it's not Pauper legal because it has never been a common.

"Currently mill is a fringe-deck" and the main reason decks like this can exist. Always going to have bad match ups, it's the nature of the game. Leyline is a good hedge against Mill and discard (like 8 Rack, etc), so while some of those are fringe scenarios, how many Leylines in the sideboard will depend the player's local meta.

Don't know abut any earlier ones, but about a month ago Wedge (the Mana Source on YouTube) brought a deck very similar to this one to everyone's attention. It came to my attention when I saw videos of Corbin Hosler (TCGplayer) playing it on TCGplayer's YouTube channel

True. Could split them to cover both efficiency and Chalice.

Nature's Claim might be better than Seal of Primordium. You don't exactly have a benefit for Seal of Primordium being an enchantment and Nature's Claim counts towards Spell Mastery for Nissa's Pilgrimage. And the 4 life is meaningless when you're Belching for 35+ damage.

Also, you probably know this, but didn't mention it: Obstinate Baloth is awesome against 8 Rack :p

double up on the meme

Firebrand Archer is a {1} cheaper, 1 less damage veraion of Guttersnipe

Not sure the Mimic + Thopter idea is goid here as you'll have to call Thopter when you play Mimic to give it a +1/+1 counter. In this deck, Mimic should be calling Construct all the time as all your non-Thopter creatures are Constructs, including Karn's token(s).

I'm not tge best of blue players, so instead of trying to give advice that may not be good, I'll point you in the direction of a cool deck that's a lot like this one, which will hopefully give you a few ideas on where to take your idea:

https://youtu.be/d_2ZTKYrpmI

https://www.mtggoldfish.com/deck/1061584#paperhttps://www.mtggoldfish.com/deck/1061584#paper

Shred Memory can tutor for two of your combo pieces, which is especially helpful since you need two Myr Retriever to combo off.

To my knowledge, Pauper (the format) means no uncommons, rares, nor mythic rares.

Helm of the Host is still worth considering. There's some other good effects you can duplicate, like making a second Shalai, Voice of Plenty to give both Hexproof and making multiple Lyra Dawnbringer / Baneslayer Angel to close the game quicker.

Some form of artifact hate, at least in the sideboard, would be good. Angels are generally large, so things like Ensnaring Bridge would slow you down / stop you.

And while I understand your frustration with Iona, I hope you don't really burn other people's cards. That would be bad.

Was just spouting whatever came to mind. Glad that at least some of my ramblings helped.

The biggest problem with Angels is the cmc on most of the good ones. I see you're trying to combat that with some of your card choices, but have you considered a Tron package?
Quicksilver Amulet is another good way to cheat them in. And I guess Aether Vial works to a degree.

Also, Iona is one of, if not the best prison style Angels. Stopping an entire colour is often a free win, and with Helm of the Host you get to eventually block all colours.

Search for Tomorrow is a favourite of mine among the land fetch spells. Goves you something to do on turn 1 (Suspend) and puts the land in untapped.
Recross the Paths is another favourite. A little slow, but makes up for that with its readability.
Khalni Heart Expedition is another I like.
I also like to sometimes run Snow lands to double up on Rampant Growth with Into the North.
Nissa's Pilgrimage may only fetch basic Forests, but with your Threshold plan, you should have Spell mastery fairly often.
Shefet Monitor cycles to ramp a land, while Greater Sandwurm only cycles. Both are faily large creatures with viable abilities early game and can contribute to Threshold.

There's an infinite mana combo with Grand Architect and Pili-Pala. With one of each, pay {U} to turn Pili-Pala blue, then tap it for {2} with Grand Architect's ability. Spend that {2} to untap Pili-Pala with its own ability and get 1 mana of any color you want. Repeat the tap and untap cycle as often as you like, then use all that mana on Walking Ballista for a combo win.

It's a card from original Ravnica block, thus not Standard legal.

Wild Cantor is a more accurate equivalent to Skirk Prospector than Simian Spirit guide. Though some people probably don't know what Wild Cantor is.