The Demo is effectively just the prologue, but the Early Access (Public) version is available in the download links or via the Early access page here: https://pinebrook-games.itch.io/project-thrust-early-access and that will allow you to play everything in 0.3.1, not just the Prologue. That includes content for Mel and Bridgette. If you want to play 0.4.0 which has content for Sammy, Carla, and Gigi, then you'll want to check out our Patreon and become a supporter for code access to the 0.4.0 page, or you can wait until later on this month when it will become public as well.
A Steam release is planned closer to 1.0.0 as Steam is stringent on Adult games getting updates and we want to make sure it doesn't get taken down just because we added new content (despite the adult nature of the game not changing).
Please let us know if you have any further questions!