Driving the South Island Without a Reservation
The most common piece of advice about New Zealand is to book everything. I think that is exactly backwards for all but two items.
Book two things, improvise the rest
Hut nights on the popular multi-day tracks and vehicle space on the inter-island ferry are the two genuine bottlenecks. Both release on a schedule, both go quickly for the good dates, and neither can be solved on the day.
Everything else, accommodation included, has enough slack outside the absolute peak that fixing it in advance costs you more in flexibility than it saves in certainty. On a five-week trip through weather this changeable, the ability to stay an extra night because the cloud lifted is worth more than a discount.
So the plan is a skeleton: two fixed points, a rough direction of travel, and a list of things I want to see with no dates attached to any of them.
The map lies about time
Distances look European and behave like something else entirely. The roads are narrow, frequently single lane at bridges, and go around terrain rather than through it. A route that a mapping application estimates at three hours will take five with the stops you will actually want to make.
I plan around three to four hours of driving a day maximum. That sounds conservative until you understand that on the west coast the driving is the attraction, and treating it as transit between destinations is the standard mistake.
The corollary is that the itinerary has to be shorter than it wants to be. I have cut two regions from a five-week plan already, and I expect to cut a third once I am actually there and moving at real speed rather than planned speed.
What the flexibility is for
Weather, mostly. The alpine and west coast weather turns fast, and a fixed itinerary means seeing the best of the country through cloud because that was the day the booking was for.
The other use is the small towns. The days I expect to remember are the unscheduled ones in places with one main street, which by definition cannot be planned in advance because you do not know which town it will be until you are in it.
That is the whole argument. Fix the two things that cannot be fixed later, then protect the ability to change your mind, because on this particular trip the ability to change your mind is the product.