The short of it was that I found the firmware for Xperia models that were sold here at one point (and hence have modem firmware containing NZ carrier configs). I was able to flash just the NZ config blobs (at least, the spark .mbn) from the Xperia 1 IV using efsTools. They're extracted and available here (I didn't post those; it's someone else's github. They're binaries, so if you want to go a safer route, I think there's a way to extract them from the stock firmware, although getting that without trusting a source-unavailable XDA tool ended up beyond me.)

Even though the chipset is different, it seems those config files are still compatible. I may have also needed the pixel IMS app to activate it once they were flashed, can't recall. The pixel tool didn't work on its own.

For 5G, I used EFS tools to modify one of the band xml files to add 5G bands to the block spark nz/oceanea uses. I'm suspicious I'm not getting both 5G bands though.

I'll write up a better tutorial at some point, but that should point the way. If you are on windows, you can get the drivers you need by signing up to the qualcomm dev program and downloading their software center tool. I personally don't recommend downloading the old qpst from the shady site everyone recommends.