You do not have to book the highest category. I often choose BF, so I can do just fine with booking the cheapest OV.
Just applied my points to an upcoming cruise - put all my negotiating skills to the test and now have a BF balcony for about half the current solo fare. Originally booked BF but rebooked onboard OK (obstructed OV) on board, so got an upgrade to OA which also included a 10% discount for 2023 cruises. Then used points to upgrade back to BF, getting a cabin 4 doors down from the original booking (this particular cruise is GTY for all balconies anyway). Not sure that the OK-OA upgrade made any difference, paperwork shows OK-BF upgrade - but I'm very happy with the end result, saving about 2.5K from current pricing.