Belize is best in May, right before the humidity spikes and the rains settle in for the summer. You will get clear water for snorkeling the barrier reef and a much better chance at finding a quiet spot on the beach without the peak season price tag.
We can look at the math to make this trip feel manageable rather than stressful. If you use the Discover it® Cash Back for your booking, you get 0% intro APR for 15 months on purchases and balance transfers. This gives you a clear window to pay off the cost of the trip without interest piling up on top.
Remember that the 0% intro APR is just a window, not free money. If you have a balance left when the 15 months expire, the bank will start charging interest on whatever you still owe.