Yea, set up Type: Null is cool too, but comes with the huge downside of lacking the flexibility that Silvally brings, which in-game can be very nice. Regarding good Pokémon you missed I can't really help, because I'm really bad at remembering when you get what. I personally am a big Ampharos fan until after you beat a certain leader in the Agate Circus (Max Spdef bulk with Cotton Guard, and maybe Charge Beam). Now just looking at your team, I'd say Silvally-Dark could be a nice fit currently. But the problem is, I don't remember when you get access to Swords Dance, but iirc it's gonna be a while. So imo just give your Silvally HP and then some combination of Def and SpD, and make it a Pivot with Parting Shot, Multi Attack, Coverage (has access to the Pledge Moves that you can teach it in the circus), and then Toxic / Twave when you get them