If I'm in conversation with a friend, I would consider using "a person I know." But if I'm in a conversation with another business person, then I would use "a professional acquaintance"

I believe I'd say "He/She and I have been professionally acquainted since 2017" I like your first sentence the most, it's pretty much spot on!

You could also say, "I have been professionally acquainted with (insert person's name) since 2017 through working at (insert employer)." If you were going to list them as a reference with their contact information. Make sure you ask them if it's okay that you list them as a reference. If they've said you could use them, let them know when you've applied for a job so that they can watch for an email or phone call from the employer :)

The first one is good. The second one sounds akward. "I have known him in a professional capacity for a decade" is a way I often say it.