A lot of "ifs" in your plan! Work it backwards. Airlines recommend you arrive at STT three hours before your flight, especially on weekends. So that's 10:30 am. Because you have carry-on only, not sure you can use a porter, which saves time.

You will need to clear out of the BVI and into the USVI, which may be on St. John. Where will the water taxi leave you? If Red Hook, you will need to catch a taxi.

Can you change to a later flight?