Garchomp - Like already mentioned in the comments, this looks like a semi defensive build, which is definitely not optimal for a sweeper. The defensive EVs, defensive item (although Rocky Helmet does stack with Rough Skin, the goal is to sweep and not get hit at all anyways.) Here:
Garchomp @ Life Orb
Ability: Rough Skin
Jolly Nature
EVs: 252 Spe / 252 Atk / 4 Def
- Swords Dance
- Earthquake
- Stone Edge
- Scale Shot
Slowbro-Galar - Because of its lackluster stats, Slowbro-Galar is not recommended to be used in OU. It's completely outclassed by other pokemon (like Toxapex) that do what it does, just better. If it's an AV user you want, Landorus-T is one of the best options in OU right now:
Landorus-T @ Assault Vest
Ability: Intimidate
Adamant Nature
EVs: 252 HP / 160 Atk / 80 Def / 4 SpD / 12 Spe
- Earthquake
- Rock Tomb
- Knock Off
- U-Turn
Most defensive sets run 8 speed, so I always give my Landorus 4 more EVs to speed creep.
Dragapult - Hydro Pump misses a lot and doesn't cover much. Flamethrower is better. (Fire Blast, if you're feeling lucky.) U-Turn is another option for pivoting, but if you're going to run U-Turn, you should probably drop Thunderbolt and keep Flamethrower over Hydro Pump.
Rillaboom - Good set, nothing to change here. I wouldn't recommend going for speed, since Grassy Glide is Rillaboom's main boon, but you could try it.
Zapdos-Galar - Close Combat is better than Blaze Kick. U-Turn is better than Throat Chop for pivoting; Throat Chop just isn't viable for anything. If you don't want to use U-Turn, Stomping Tantrum is another option.
Magearna-Original - Timid > Sassy, not sure why you chose Sassy when it lowers speed, instead of lowering Attack, and you don't have any physical moves. 252 Spe over 252 SpD; yes, Magearna's speed is low, but I think you should run a shift gear set. If you don't want to, 252 HP is better than 252 SpD. 0 Attack IVs for minimum confusion damage. I'm not too familiar with Magearna-Original, so if that's impossible just ignore that. It doesn't really matter too much anyways. Shift Gear over Volt Switch.
