Megas don't exist in gen 8. Also, you are missing a move on excadrill as well as having sand force but no sand setter(if you did btw sand rush is better). Gardevoir is hugely outclassed in ou, by hatterene. A modest bature, 252 hp and sp atk, and 4 Def with calm mind, draining kiss, psyshock, and mystical fire us better, with leftovers(I'll post the set at the bottom). Rose Rade is decent as a toxic spikes setter, but it's a bit frail so you may want a sash, considering it's a suicide lead. Also, because of this you may want sleep powder and spikes(again I'll post the set at the bottom). Choice items and setup moves are a no no, cause you can't attack if you setup. Band gyrados isn't great, so again I'll put the set at the bottom. Choice band Salamence isn't the best, it gets the powerful moxie as well as dragon dance, so I'll use that set. Finally, blaziken is missing a move, and close combat over>brick break.
Revised movesets:
Blaziken @ Life Orb
Ability: Speed Boost
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Protect
- Flare Blitz
- Close Combat
- Earthquake
Protect to gain speed. Flare blitz is strong stab. Close combat is as well, and lowering defenses doesn't matter because it's already frail. Earthquake so you don't miss out on needed coverage against toxapex.
Salamence @ Heavy-Duty Boots
Ability: Moxie
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Dragon Dance
- Earthquake
- Outrage
- Dual Wingbeat/Roost
If you want a choice item you can do that, but I find dragon dance and moxie sets easier to use. Dragon dance gets a speed and attack boost, earthquake covers a lot, outrage is very strong stab. Dual wing beat is just there, bit it's replaceable by Roost for recovery.
Gyarados @ Heavy-Duty Boots
Ability: Intimidate
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Dragon Dance
- Waterfall
- Power Whip
- Earthquake
I decided not to go for moxie here, as 2 all out attackers wouldn't be so good. Intimidate makes the opponents attack drop, and it can survive a bit more this way. Dragon dance is setup, waterfall is stab, earthquake for coverage, and power whip also coverage. Also, I think you should have a bulky-er pokemon or a wall, so here's a toxapet set because your team has no walls.
Toxapex @ Black Sludge
Ability: Regenerator
EVs: 252 HP / 156 Def / 100 SpD
Calm Nature
IVs: 0 Atk
- Scald
- Baneful Bunker/Toxix(with tornadus)
- Recover
- Haze
Scald is to burn. Normally you'd have toxic or toxic spikes, but Roserade does the job, so baneful bunker is to stall them with it. Recover to heal, and haxe to not get setup on.
Roserade @ Focus Sash
Ability: Natural Cure
EVs: 252 HP / 4 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Sleep Powder
- Spikes
- Toxic Spikes
- Leaf Storm
Sash to sleep them, then spikes and toxic spikes to hurt them with hazards. Leaf storm also blows away swampert, another common stealth rocks setter.
Excadrill @ Life Orb
Ability: Mold Breaker
EVs: 252 Atk / 4 SpD / 252 Spe
Jolly Nature
- Rapid Spin
- Earthquake
- Iron Head
- Rock Slide
I considered replacing this but having hazard control is nice. Rapid spin is for that, iron head is stab, so is e quake, and rock slide is coverage.
Hatterene (F) @ Leftovers
Ability: Magic Bounce
EVs: 252 HP / 4 Def / 252 SpA
Modest Nature
IVs: 0 Atk
- Draining Kiss
- Mystical Fire
- Psyshock
- Calm Mind
Draining kiss heals more, calm mind is setup, mystical fire is coverage, psyshock is stab and can help again blissey. I also want to note that this could and should be changed. Unfortunately, you have lots of physical pokemon, so I am going to change excadrill and hatterene(but you can use those sets too, these are just a bit better)
Tornadus-Therian (M) @ Heavy-Duty Boots
Ability: Regenerator
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Defog
- Hurricane
- U-turn
- Heat Wave
Also packing hazard control, but unfortunately also clearing the opponents. Hurricane is stab. U turn can heal it cause regerator. Heat wave can also eliminate steels that block hurricane.
For a special sweeper, spectrier works well. I have 2 varients, choose which one you want.
Spectrier @ Leftovers
Ability: Grim Neigh
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Substitute/Will-O-Wisp
- Nasty Plot
- Dark Pulse
- Shadow Ball/Hex
This one is a sub-plot, and can let you setup safely. Shadow ball and dark pusle cause of its sad movepool. It also gets stronger with each ko thanks to its ability, which raises sp atk when it kos something.
You can also do will-o wisp instead of sub and hex instead of Shadow ball because hex is strong when they have status.
Spectrier @ Choice Scarf
Ability: Grim Neigh
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk
- Foul Play
- Hyper Beam
- Dark Pulse
- Shadow Ball
This setspeeds even regieleki because of scarf. The moves are there because it has basically nothing better.
And if it needs clarification, you'd be using Blaziken, Salamence, Toxapex, Roserade, Tornadus, and Spectrier. Also, I want to add that it's not an amazing team, because you'd need to change a lot more, and too many changes to the team is no fun.