There is no upgrade benefit as a platinum member for award bookings. If you really want a specific type of room, perhaps this is not the stay to do on points but to pay for

You could phone the hotel and see whether they will accept payment for a better room; some will and some won't

You could wait, since despite there being no benefit, some hotels may upgrade even on awards

Just booking multiple nights , some paid and some not, you could end up having to move , especially if the hotel has had a lot of cases of people trying to get free upgrades on awards by doing this method; you could be lucky and get the upgrade for the whole stay