I’ve read both Parable of the Sower and Parable of the Talents, and while you can tell the general arc she was going for with the next book (and can imagine the broad strokes of what would happen next), I never felt there was a lack of closure for the story.
They are emotionally difficult books to read, so it’s also hard to recommend them to people, but I would encourage you to not let the lack of a third book prevent you from reading the first two, they are worth reading on their own merits. The Parable of the Talents especially has significance to the situation in the U.S., as some say it predicted Trump.