What about Greenhouse? The one in P'burg looks large enough to accommodate with rsvp. Don't know if the Greenhouse in Simpson Bay is as large.

Enold's in Grand Case has room for 22. It's casual caribbean food inside, on porch or on the beach. I've seen boat tours stop there and they have large amount of people. But rsvp.

Sol e Luna in French Cul de Sac, has garden seating that would handle 22 people. Inside has mostly small tables. It's very upscale expensive but delicious food. Maybe once during the trip. You won't be disappointed.