I'm used to swapping my team in and out, though depending on where you are in the game, you may not be able to get some of the pokemon I recommend. The last time I did charlotte's gym, I had a Tyrantrum with Earthquake and Rock Slide, an Archeops may fit your team nicely too, with typing and tailwind/stealthrock support. That said if you're already at Agate Circus, then you can't go back to revive a Tyrantrum, since that requires going to Spinel Town. Tyrantrum at least provides some heavy hitting, although you'd need to pull off some breeding shenanigans to get one with Dragon Dance.
If fossil pokemon are currently unavailable though, Lycanroc can help. You can near the Fiore mansion use fishing to get a Horsea which you can evolve into Kingdra for additional type coverage, and also you will need a surf-compatible mon soon enough. (Hope you picked up the Good Rod though, you'll need it to get the Kingdra.)