If you purchase once you get all future updates including the full release when it's done. I've just been replacing the link in the downloads section with each new version. If it isn't giving you the download link then the purchase probably isn't tied to your itch account. You can either try to track down the original email to the download links if you have it or get in touch with itch support to recover the purchase and link it to your account. - https://itch.io/docs/buying/already-bought