If you are willing to pay the current price, the best strategy for a
Disney Cruise is to book as soon as you have decided to take the cruise. As long as you are not booked in Concierge, you can cancel for a full refund up until the final payment date if you decide to book a restricted discounted fare or take a different cruise.
Once you've booked, you can watch to see if discounts are announced.
Some discounts (such as the one for
Disney+ subscribers a few years ago) can be applied to existing bookings. If these discounts become available, you can take advantage of them and keep your room assignment and any onboard activities and excursions for which you have registered.
Other discounts are valid only for new bookings and require a non-refundable payment in full at time of booking. These restricted fares also do not allow you to choose a room -- you must take whichever room
DCL chooses to assign you within the category you book (Inside/Oceanview/Veranda) and you may not get a room assignment until shortly before sailing.
If you're not willing to pay the current price, your best bet is to keep watching for discount offers and see if they result in a price low enough for your liking. Restricted fares are most likely to be announced after the paid in full date (and after most passengers have already booked onboard activities) but do sometimes come out earlier.