If you use a TA you will normally get more OBC.
Multiple thousands of dollars on a full world cruise with Crystal in a penthouse. We used it all on excursions. Wonderful cruise agent made it possible. She is with Avoya which is good if you use Amex Platinum as they are related. We got double MR points at the time. PM me if you have Crystal questions. No I am not an agent myself only a cruiser. Cruising is the last hold out for using TAs. If you don't use a TA or at least a TA agency and book direct you are leaving lots of money on the table. Friends don't let friends book directly with cruise companies.