i know its a casual deck but you have way too high of mana cost in everything, for one Dark Depths is not meant in this kind of deck if you dont have a way to cheat his counters or have the mana to take them out, as well as Sigil of the Empty Throne you dont have enough enchantments, might as well take them all out. Tempest Djinn, Angel of Jubilation, Yosei, the Morning Star, Exquisite Archangel, Goliath Sphinx, Ornithopter, Rustwing Falcon, Serra Ascendant, Azorius First-Wing, Resplendent Angel are either to expensive, you dont have a reliable source to use they'r abilities or are simply underwhelming. for lowering the mana cost i suggest Warden of Evos Isle. you can replace Serra Aviary for Favorable Winds, for counters Spellstutter Sprite, Nimble Obstructionist, Spell Queller,Siren Stormtamer and probably not very good if its not a fast pace deck (storm) Erayo, Soratami Ascendant. Hope of Ghirapur and Selfless Spirit are good protection, Vendilion Clique and Gilded Drake some good control. Palace Familiar, Owl Familiar net you some card draw and Illusory Angel, Sky Spirit seem ok. i didnt go in to much noncreature spells and i lck sleep so i hope i was helpful.

Thanks a lot Dete for this comment!

I see I can basically rebuild it entirely ^^

As for mana costs, I didn't have a lot of lack of mana issues when testing the deck, although there are definetely some high-CMC cards I've never actually got the chance to cast indeed.

For this deck I'm still unsure if I should do a more efficient 4-of-of-the-best-Flying-cards deck, or rather a more casual deck, with most cards put only once. There are 2,400+ cards referencing this keyword, nearly half of them W/U, so it feels like a waste to put only a few different cards in it.

Anyway, will see, thanks!

if the thing you want is to have a deck focused on flying creatures or spells without good or bad additions, why not make a commander deck? if you already got the colors figured out you could use Raff Capashen, Ship's Mage, Azor, the Lawbringer or even Medomai the Ageless. with that combination you got a lot of ways to control the board and protection for your creatures, and add 40 more cards to the deck. Aboshan's Desire, Flood, Tidal Flats, Spirit Bonds, Invocation of Saint Traft, Angelic Favor, Entreat the Angels, Tidal Surge, Decree of Justice, Battle Screech, March of Souls, Descend upon the Sinful, Storm Herd are cards that you could use in commander but not in this deck... if you want to have a reliable way to trigger Sigil of the Empty Throne you can use Flickering Ward, Shimmering Wings, Conviction, Brilliant Halo, Gossamer Chains, Viscerid Armor, Ephara's Enlightenment and others. Aven Mindcensor, Mana Short, Boomerang, Standstill, Magus of the Moat, Bident of Thassa, Sigil of the New Dawn, Hindering Light, Supreme Verdict.

                                                                                                             ooor make a pauper deck edh or a normal one... :P