Whether there will be a path for the MC to become one of the Phantom Thieves will depend on the writers in charge of that route, so unfortunately I can't promise 100% you'll be able to do that... BUT with that being said, we'll probably provide a walkthrough for any completed routes in the future, so dw about not being able to 'fully complete' the game or anything like that!
I'm so happy you enjoyed the demo enough to drop us this message! Thank you dear ♡