Rate my national dex ubers team. What can i improve?

[fLaT_eArThEr.global]

Diancie-Mega @ Diancite
Ability: Magic Bounce
Tera Type: Rock
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Diamond Storm
- Play Rough
- Stealth Rock
- Sandstorm

pancake snake (Zygarde) @ Leftovers
Ability: Power Construct
Tera Type: Normal
EVs: 252 HP / 252 Atk / 4 SpA
Naughty Nature
- Thousand Waves
- Glare
- Extreme Speed
- Coil

magik whale (Kyogre-Primal) @ Blue Orb
Ability: Primordial Sea
Shiny: Yes
Tera Type: Ground
EVs: 252 SpA / 6 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Origin Pulse
- Thunder
- Ice Beam
- Scald

sword doge (Zacian-Crowned) @ Rusted Sword
Ability: Intrepid Sword
Shiny: Yes
Tera Type: Water
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Behemoth Blade
- Swords Dance
- Wild Charge
- Close Combat

sneaky (Marshadow) @ Marshadium Z
Ability: Technician
Tera Type: Normal
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Shadow Sneak
- Spectral Thief
- Close Combat
- Bulk Up

bacon bird (Yveltal) @ Black Glasses
Ability: Dark Aura
Shiny: Yes
Tera Type: Dark
EVs: 4 Def / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Dark Pulse
- Oblivion Wing
- Heat Wave
- Taunt

 

[awyp]

I am locking your RMT for the time being as the forum rules require that you provide at least three lines of descriptions for each Pokemon to explain their roles. Please read the rules carefully and look at Team Showcase to see the detail we require. PM me with updated descriptions, and I will be happy to unlock this. Also, feel free to PM me if you need help or have any question. Have a nice day!

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Why did you choose this Pokemon?
What do the EV spreads do and why do you have it?
What are all 4 moves for?