Make every team member hold a Babiri Berry, except Primarina. Give Primarina a Wacan Berry for Flygon's Thunder Punch. Surf's damage is lowered in Double Battles because it targets multiple opponents, so if it doesn't OHKO Gigalith (it'll get Sandaconda, but Gigalith gets the Special Defense boost from Sandstorm), Aura Sphere it with Togekiss and switch out Primarina.
Dynamax Togekiss and attack Duraludon with Max Knuckle (Togekiss) and Drill Run (Rapidash-Galar). If it doesn't KO, you'll probably be fine, and you can KO next turn.
It probably won't be nearly as complicated as this. Raihan is actually fairly easy due to EXP Candies and SwSh just being general pushovers, but your plan is solid and you'll be able to pull through with no deaths.