Working on a report but I headed over for 2 weeks in May. The Traveller Declaration is pretty straightforward. I took a picture of my CDC vaccination card and emailed it to myself. No issues uploading it. You can start the declaration up to 28 days before you travel. Once you create it, they'll email you instructions I think with a link to resume the form when you have other information to add.
The biggest pain in the process is the covid test. I don't know what your flight plans are but it seemed like most of the flights leave in the evening which for me meant doing the RAT same day. I had trouble finding a PCR testing option that would guarantee you results within 3 days let alone the 48 hours required by New Zealand.
I flew out of Chicago connecting through LA. The Chicago airport had a testing service which worked remarkably well. Really long story but my friend who was traveling with me ended up needing to fly out the next day so he got a PCR test and they had his results back to him in not much longer than it took them to get me my RAT results. I think if you can find a place that doesn't send their tests out to an external lab that it can be done PCR which would give you more time and less anxiety about getting the results.
Once I had the testing results, I was able to upload them to complete the Traveller Declaration and submit it. I don't remember exactly but it took them probably 15-20 minutes to approve it. I don't know if that's the norm for everyone, my friend said it was about the same for him but that's just a sample size of 2. You get an email with a QR code that is used by your airline to prove that you completed the declaration. Checking into the LA to Auckland flight was the only time that I needed the QR code the entire trip. I don't know if they'll have a better system worked out by the time you travel but Air New Zealand manually checked everyone on the flight in at the desk at the gate needing the Traveller Declaration QR code and the NZeTA. I had been so focused on the Traveller Declaration that I had forgotten to do the NZeTA before the same day. This was also approved in 20-30 minutes but I would recommend doing this well before, just not worth the anxiety.
I hadn't thought about it but my domestic leg was codeshared between Air New Zealand and United. United wanted to independently verify the vaccine card and testing results that Air New Zealand would require. They were slower than New Zealand in approving the documents on their online system and I ended up getting help at the United desk to check into my flight. I showed them my vaccine card, test results, and what I told them was my NZeTA (I hadn't actually gotten that approval email by that point) and got my boarding pass. If you have a separately booked domestic flight, this shouldn't be a problem but thought it worth mentioning.