Booking flights only when final payment is due can be risky, depending on where the cruise starts and finishes.
For us, it it includes a long haul flight, we often use Avios points to upgrade to Business Class. In order to get BA ‘reward’ flights you usually need to book as soon as flights are released (355 days ahead).
For short haul flights we need extra leg room seats. My DH has a spinal problem, so can’t use exit row seats. Therefore there are few seats we can book and these tend to get booked very quickly so we always book our flights as soon as they are released if possible.