Sol is fine but I'm not sure she'll fit in a group with Osiris. She'd complement Cthulhu, but Dragon King or SSR Nike would probably be better picks if you wanted to focus on Water. I recommend starting with Sol since it minimizes the rerolling required, as she can fit into random builds based on what the Gatcha gives you. Since you rerolled for 2/4 of an optimized team, Sol is less valuable to you. You should further optimize your best team, not generalize.

For the Dark build with Osiris, you'll want to use Mordred with Sniper Shot. That means picking up D'art next. Mordred will help you land your debuffs, and Black Propaganda is indispensable for most content.