This is hard to answer because when it comes to "hacking" everything is possible so short answer, yes there's ways to hack your phone. How hard it is? Insanely hard IF they don't have your passcode, if they somehow manage to unlock your phone then installing keyloggers or stuff like that would be really easy. What are the chances of this happening? Very low, there is a lot of tools out there that do everything automatically but it's very hard to find them and know how to use them and in my opinion, I highly doubt it the phone fixing guy knows how to "hack" your phone.