Some suspension modifications and maintenance!
The front creaks over bumps. I noticed the tie rod end boots were cracked, and decided to do tie rod ends as well as links. Fortunately, USDM and JDM parts match. Unfortunately, these did not fix the creaking. I'll return to this at a later date.
The rear springs in these cars are considerably softer than the fronts (2.0k in the front?), and most of them are frequently slamming the rear bump stops, even with no weight in the back of the car. The popular spring upgrade for these cars are from a 98-05 Daewoo Matiz, which results in (I believe) a spring rate increase from 1.4k to 2.1k in the rear. This also makes the cars understeer less, and reduces body roll when cornering. However, I could not source any Matiz springs in New Zealand, and shipping from Europe was going to be 5x the cost of the springs.
After poking around in a scrapyard, I found 03-13 Mazda 3 rear springs were the only match in terms of coil diameter and spring rate. Uncut, they're ~3k springs, but they need to be trimmed by almost 100mm for the car to sit at stock ride height in the rear, unloaded. Trimmed anymore, and they would get loose in their seats with the rear in the air, requiring different (shorter) dampers. Trimmed to this length, math tells me their spring rate is now closer to 4k. I expected the ride to be punishing, but they're severely underdamped with the stock (likely tired) dampers, so instead they're firm, a touch bouncy, but not harsh. They also make the car handle almost completely neutrally in turns.
They also have the added benefit of still having plenty of suspension travel, even with 12 bags of cement in the back of the car.