Am I ready to take on Oplecid's Gym?

Yes, with all those Pokemon at an exceptional level, you should be able to beat them. You could add a temporary Dragon or Ice type move to help beat Iris/Drayden although you probably don't need to. For Simisear, Will-o-Wisp would be useful since Iris'/Draydon's mostly use physical attacks so you would be able to cripple that. Although Defeatist isn't the best ability, it should be okay for playing through the story.