A literal reading of Twelver Shi'a hadith indicates that Allah created and appointed the prophet as a Prophet prior to his existence in this world. So it was not due to activities that he chose to do, such as mysticism, that he became a prophet. 

Of course, he had some practices which can be called "mystical" such as spending time alone in the cave of Hira for worship. It is natural that someone who is very close to God would want to choose to do those activities. 

My understanding is that the time "before" this world is outside of chronological time, and therefore Allah did this with full knowledge of what was before and after. So therefore Allah knew the Prophet's merits over all people, rather than just only pre-designing him as the Prophet. That is, it is a combination of the individual merit of the Prophet and the creational will of Allah. This was why he was born with 'ismah; otherwise, it might be unfair for Allah to grant some poeple 'ismah and not others. Other people may have different understandings. But in any case it was not solely due to the Prophet "earning" the prophethood through ascetic or mystical practices.

However, some other people, including some people who are not Muslims but who feel the Prophet was close to God, might see it this way and might agree with your view.