dkenny, we're in the "same boat" - sort of :-)
We arrive on STT Sat 4/15 after the last ferry as well. Staying at Island Beachcomber that night; ferry to Tortola on Sunday (Easter) to stay in CGB at an Airbnb for a few nights before our charter.
I hadn't thought about the ferry schedule on Easter - do they have a special/limited schedule for Easter?
Last trip we rented a car through one of the national chains and the rental agency was inside the lobby at Maria's. However, you need to be specific about what time you're picking up your car. We waited 2 hrs in the lobby for our car to become available because we got there early. A local rental place might be more accomodating.
Or pre-arrange a taxi - we've used AC Taxi service (Arnold is a sponsor here) and he was great. Large, clean, air conditioned van with plenty of room for a family + luggage. Sometimes the taxi is much easier than a rental.