Hi, thank you for playing our game!
Sorry we've missed your post, sometimes itch.io doesn't notify of new topics in Community page. Third boss is more of a proof of concept and provides a rather steep challenge for players as there is not much content after it. We'll be improving the boss and adding more content for the full version.
Good news is that upcoming update will change how pricing for Zooids works and ungrowing will be made to return more than a base of Zooid cost. This will promote experimenting and will make mastered levels act like a sandbox mode. Stay tuned!