So, I have a smallish updated.
First, I've finished sweating PEX fittings on the rest of the panel inserts, yay.
Second, I've been pressure testing the panels. This isn't the fastest process, but IMO its necessary before putting them up in the air and finding out there is a leak!
To do this, I crimp plugged PEX lines to each corner. One has a air fitting with a ball valve and pressure gauge. I fill it up and let it sit for a while.
This part is all easy. The hardest part is actually removing the PEX lines. Once they're crimped on there they are very hard to pull off, even after cutting the metal crimps off. If anyone has any tips on how to remove PEX lines from then fittings it sure would be nice.