Flights to Manila are $1400pp in December. Plus all the better diving is another flight elsewhere. Not sure you could do that and stay within budget although I understand accommodations are very affordable.
I'd look at places where you have access to multiple islands reasonably in the Caribbean. Maybe the USVI's/BVI's. Since I gather your wife thought Utila too boring for a month, if you were willing to move around a little you could see a lot of different things.
An advantage to the USVI's/BVI's has to be the fast ferries. Makes getting to multiple islands reasonable. Plus for you, St. Thomas is a $500 r/t flight in December. (from Chicago)
From St. Thomas you can be on St. John in under 1/2 hr. - even bring the rental car - there's a car ferry between the two. Tortola is about an hour and St. Croix in (I believe) under two. There's a few shore dives on St. Thomas and boat diving both on the quieter East End or south of Charlotte Amalie. We stayed out on the East End near Red Hook for a few days, it's pretty "chill". Most condo rentals are either beach front or a short walk. From our courtyard we could clearly see the beach and the Cays out towards St. John. There's about 1/2 dozen good boat dive operators in that area.
Then there's St. John. Pricier so maybe just do some day trips from St. Thomas. There's more shore diving there but not a lot - I believe about 4-5 sites. And good boat diving in the marine reserve that covers part of the island. Low Key Watersports there also does trips out to the BVI's - mostly the Rhone. The early ferry from St. Thomas gets you there in time.
Then maybe on to St. Croix - there's a dozen shore dives there off either the Cane Bay wall area (north side) or the Frederickstad Pier area. Lots of condos on the north shore and even the Divi Carina Dive resort is an option - although might be more expensive.
Lastly head over to Tortola from St. Thomas. Mostly boat diving from there, you could access the Rhone, Jost Van Dyke, possibly Anegada plus all the really good diving in the smaller islands across the channel south - Salt, Cooper, Ginger Islands etc. The Rhone wreck is out there - Low Key Watersports also dives it from St. John weekly. If you catch the 1st ferry in the morning you can get there from St. Thomas in time for the departure - it's an all day excursion. We stayed on Virgin Gorda which was smaller, quieter and pricier. I've been to Tortola but only the Roadtown area. If you do wind up on Gorda, DiveBVI is one of the best dive operators I've used in the past 20 years.
Also look closely at the ferry schedule. Although we ran out of time, I had planned two day trips to smaller islands that would've returned the same day.
Some info:
Home | US Virgin Islands
Virgin Islands Ferry Schedule
Scuba Shore Diving Site Listing for: USVI, St. Croix, Caribbean
BVI Scuba