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This core seems solid. You'll want lots of switching moves to help get these powerful breakers in, so you absolutely want a Landorus-Therian. It can either be Defog or Stealth rock depending on how the rest of your team goes. You're going to use the rest of your team slots covering yourself defensively, since Gzap and Toxt have no defensive utility.

Just make sure whatever defensive mons you add to Lando / Toxt / Gzap, that none of them suck momentum. Eject Button Toxapex, Rotom-Wash, Slowbro, and Tornadus are some great defensive pivots. Pokemon like Ferrothorn are also good because they threaten a lot of switchins with Knock Off, SR, and Leech. You want to AVOID stuff like a DD Dragonite. Hit or miss, sucks your momentum, often forced to use roost, etc.

I was thinking a bit before I posted the initial message and thought about Lando (defog) heatran (rocks) and Slowbro.

Will I need a breaker for the final slot? Or another defensive pivot?

Between Gzap and Toxtricity you have sufficient breaking power. There's no way you could support another breaker.