Kingdom Hearts III gave people a good jumping off point. Yes there are sequel hooks and mystery boxes. But Sora defeated Xehanort's army and saved his girl from the jaws of death. He even says before the credits that his whole journey was all for her. For all intents and purposes, the saga is complete.

IV is in development, but with the Missing Link gacha being cancelled and Nomura talking about his eventual retirement, I don't know what the future looks like.