Oh, completely. These days almost anything goes depending on the individual and how well you know them, who they work for, who they are. I’ve had clients with more initials that the alphabet and get one of the dots wrong and that’s six figures business in jeopardy. That’s knowing the client...
Formal = Dear Sir, yours faithfully or Dear John, yours sincerely e.g. in your situation, that’s appropriate.
Informal = Hello John, regards (if don’t know person yet)...all this ‘kind’ regards, ‘best’ regards is really naff and unnecessary. It’s become normalised though.
Very informal = Hi...