Yes you more clearly need to define downtown as Manila is basically an agglomeration of several cities. There is A Sheraton in Malate that is usually much cheaper than the Airport Marriott or Sheraton. At that time of night its around a 25 min drive from the airport. As a Platinum you should be able to get 4pm checkout from these hotels.

Daytime travel in Manila is very problematic. It is literally faster to walk much of the time so best to select a hotel near where you want to be. Traffic usually eases off after 9 pm. Getting a Grab at the airport is very frustrating. The Grab stand has long queues and dialing into the app is problematic as the taxis are not allowed to stop there if you select a grab taxi which is usually the cheapest. Touts there are over the top on pricing so for convenience you may want to book a car service.