If you're going to making Sun team, try to use Pokemon that get benefit from Sun. And like Mikey said, you should use a Drought Ninetails for permanent Sun instead of having Venusaur to set up Sunny Day. IF you can get Solar Power Charizard, it'll be even better along with a Chlorophyll Venusaur.
You can replace your Swarmpert with a Drought Ninetails if you want because Swampert is 4x weak to grass type attack so beware as your opponent might take an advantage from your sun and using Solarbeam on you Swampert. And you said you're usually do double battle, so replacing Charizard's Earthquake with Protect is even better.
Why you don't need an earthquake? Because Charizard's weakness are water, rock and electric attack which you can easily beat them with Flamethrower and Solarbeam. If there's an elevtric Pokemon try to bring down your Charizard, just switch with your Garchomp.
Sorry I can help you more because I don't know what your other Pokemon movesets / ability / stats. Well hope this can help you a bit at least