Busing from Cancun

Have you considered doing the bussing part on the Vancouver end of the trip?
Vancouver Rail Station (XEA) to SEA is about 135 miles. I have no idea what that cost.  Southwest Airlines flies seasonal out of SEA via DEN and all year and out of HOU to BZE. They have periodic sales system wide. Southwest has a monthly price calendar with the lowest fare each day, to help find the best deal.

looking further, I see WestJet flies out of YVR via YYZ to BZE on December 10th for as low as $245.

If it were me, I would look at options like these WAY before I would subject myself and wife to a Mexican cross border bus trip. 

ADO has frequent busses from Cancun to Belize. The buses are air conditioned and nice some even have movies available. Check their website for prices and schedule. I have heard that some people have had items stolen while on the busses. It was items in their carry on bags stored in the overhead compartments so you should be vigilant with your belongings. Cancun is about 5 hours from Corozal by car. Corozal to BC is another 2-2.5 hours. While it’s typically cheaper to fly into Cancun it’s pretty time consuming so that’s the trade off.