With TMM's exit from Belize, there are no charter companies per se in San Pedro anymore. There are a number of smaller operators who mostly do day sails
. However, I think Felicity Sailing will do multi-day trips. "Felicity" is a Lagoon
38 which was in the TMM fleet, but is now run by an individual owner/operator.
38 is small for 10 guests, but if you are OK with ad hoc sleeping arrangements then you may can work
something out. Or, Horace (the owner/captain) may be able to line up a second boat.
Another option may be Belize Sailing Vacations, they are based in Cucumber Beach Marina near Belize City. They have multiple larger catamarans in their fleet and only do full service multiday charters.
The only other charter companies in Belize (Moorings, Sunsail, Dream Yacht Charters) are all based in Placencia (too far south to be practical for an overnight trip from San Pedro).
Re stops between Ambergris and Belize City:
Caye Cauker. Its grown a lot since its days of just a few sand streets, but has kept that funky little island vibe (unlike San Pedro).
St Georges Caye. Historic site, interesting locals island, good diving
Drowned Cayes. Very protected anchorages
, good dinghy
exploring (mostly uninhabited mangrove islands though).
Geoff's Caye. A day stop, only if the cruise
ships are not in (they flood Goeffs with their hoards), beautiful little island with good snorkeling.
Another option if the charter doesnt work
out is to book a night or 2 in San Pedro or Caye Cauker then take a water
taxi into Belize City from there. This gets you an overnight in a cool place and a long boat ride in Belize.