You have sun so you should have Solgaelo. You can get a cosmog which you could evolve into cosmoem. That might be all you want. I'm doing a living dex though so if you want to have all 3 you you would have to get a second cosmog which would require you to either restart the game, get a second copy of the game or trade with someone.
Same thing for the silvally, you get the Type: Null at the end of the game at the Aether Foundation garden I believe and you can evolve it to silvally but then you won't have the Type: Null for the living dex if you're doing one.
To get cosmog, I put Solgaleo in my party and went to the lake of Sunne. There's a portal at the top of the stairs. Click on it and it should say something and turn your game from day to night. Then go to the lake of Moone and the cosmoem should be there.