Addressing a faculty member I interviewed
If the emails are for some kind of 'official' purpose, then there is no harm in mentioning "Dr.". The person, if he feels uncomfortable, then he would reply back with a note asking not to be formal. This has happened with me quite a few times.

If the email is not for official purpose but for starting a conversation in a written manner, then go with "Hey X" or something similar.

Anyway, I would not overthink on these aspects. I would always go with the safe path by mentioning "Dr.".