How to set Title in Contacts but not display it?
Set it as a Nickname instead. Then it would only be seen by you.

Don't hand the contact card to anyone, though, as it would be in that.

See Does a Contacts Nickname transmit to anyone else; iMessage or email? for a cautionary tale ;-)

A friend of mine has been "Young Master Johnson" [fake name] for 15 years without anyone else knowing.