If you have enough miles for an 'all miles' trip, then in general you should get yourself on the plane (whether than be WT+ or CW or even First) and then shuffle around as necessary. We had WT+ to Barbados (back CW) booked last year and switched the outbound about a month before departure when it opened.

If paying cash, you also need to consider if you paying a decent price. Sales come and go, we are off to Dubai in J next week for £1,199 (£1,000 for the kids) which we booked in the sale last week. Price was so low we cancelled redemption tickets and rebooked. You can't do that if you've already paid for WT+.

Caribbean routes do come up a lot in the BA sales, because they are leisure routes and demand is more susceptible to the health of the economy.

Also, of course, how flexible are your dates? If you MUST go on a certain day due to work commitments then you may be happier tying yourself down with a cash ticket now. If you can shuffle your dates based on availability, it may be better to hold off.

PS. Also consider redemptions to Miami and cash for a connecting flight, depending on price. Not ideal routing but its a back-up whilst waiting for a direct redemption.