I think this is a very good general plan, but I wanted to suggest 3 things I think you missed with this plan:
- The Statolith: An organ for detecting gravity/keeping balancing; also called: the Statocyst (Xenoturbella, Cnidarians, Ctenophorans, Echinoderms, most if not all Conchifera (Shelled Mollusks (Including Octopods and Slugs)), and Crustaceans, amoung others), the Statocyte (Plants), the Otolith (Vertebrates), and the Müller Vesicle (Loxodidae)
- The Holdfast: Used by Algae, Mature Sponges, and Juvenile Cnidaria, as well as Corals, to anchor themselves to the seafloor (auto evo might need this even if the player doesn’t want to be sessile, Algae and Coral are important to the ecosystem)
- The Water Vascular System: A type of Circulatory System used by Echinoderms for locomotion, food and waste transport, and respiration
Edit: Also, wouldn’t at least some colonial organisms basically be really big multicellular creatures? Could the caste system handle that? For example, the Portuguese man o’ war starts as one type of creature but grows into a collection of I believe seven types assembled in a way that can be mistaken for one really big one, like a multicellular organism budding more cell types into one larger creature. I am not sure if they all work that way, but that seems like something that the life stages/castes system might be able to handle.