Arcanine's defenses aren't necessarily the best, but it is outsped by other common Pokémon, so a Pokémon with Sticky Web could be useful. If you want to keep SR, then a good option would be Shuckle. It's defenses make it difficult to get off the field and a standard stall set could suffice. If you don't mind getting rid of SR then Galvantula would work.
Shuckle @ Leftovers
Ability: Sturdy
Tera Type: Fairy
EVs: 252 HP / 60 Def / 196 SpD
Bold Nature
IVs: 0 Atk / 0 Spe
- Sticky Web
- Stealth Rock
- Protect
- Toxic / Infestation
Standard support for slower allies like Arcanine. Toxic let's it deal good damage while it tanks hits.
Galvantula @ Focus Sash
Ability: Compound Eyes
Tera Type: Water
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
IVs: 0 Atk
- Sticky Web
- Thunder
- Energy Ball
- Volt Switch
This set lets Galvantula provide support and deal respectable damage. Thunder's accuracy is increased via Compound Eyes, Energy Ball kills off Ground types and Volt Switch is for a fast pivot.
Now for the new Arcanine.
Arcanine @ Choice Band
Ability: Intimidate
Tera Type: Electric
EVs: 4 HP / 252 Atk / 252 Spe
Jolly Nature
- Flare Blitz
- Extreme Speed
- Wild Charge
- Play Rough
A Choice Band Wallbreaker set. Flare Blitz for STAB, Play Rough for Dragons, Tera Electric Wild Charge for bulky Water types like Vaporeon, and E-Speed for revenge killing.
For Gliscor, I'd recommend you minimize it's Speed IVs and give it a -Spe nature, just so it can work well against Trick Room teams and be more effective as a slow pivot. Aqua Tail can be swapped out for SD to sweep in Trick Room.
For Quagsire, swap out Surf for Recover. This helps improve survivability and you still have Earth Power for STAB with Ice Beam for Flying types.
I don't see anything wrong with Gardevoir.
For a second sweeper, I have three options for you: Alakazam, Haxorus or Blacephalon.
Alakazam @ Life Orb
Ability: Magic Guard
Tera Type: Dark
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Nasty Plot
- Psychic
- Dazzling Gleam
- Shadow Ball
All moves are buffed by Life Or with no drawbacks thanks to Magic Guard. NP for set-up, Psychic for STAB and the other two for coverage.
Haxorus @ Muscle Band
Ability: Mold Breaker
Tera Type: Fairy
EVs: 252 Atk / 4 SpD / 252 Spe
Adamant Nature
- Close Combat
- Dragon Dance
- Dragon Claw
- Iron Head
Standard set-up sweeper. 2 coverage moves and 1 STAB.
Blacephalon @ Choice Specs
Ability: Beast Boost
Tera Type: Grass
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
IVs: 0 Atk
- Fire Blast
- Shadow Ball
- Psyshock
- Hidden Power [Grass]
Fast specs with powerful STAB and coverage. Tera Grass boosts the power of Hidden Power.
Out of the 3, I recommend Haxorus the most.