What should I do about a tensioned/trued wheel that lost significant spoke tension after first use?
Yes, it’s normal for spoke tension to decrease when the tire is inflated.
First of all I’d use much more tension and try to get it more even. Currently you have about 30% difference between slackest and tautest spoke, which is a lot (I’d aim for ~10%, but I’m a perfectionist in that regard).
540N on the slackest rear spoke before you’ve even inflated the tire is very little. I’d get the drive side spokes to slightly over the allowed 1200N and hope that the non-drive side ends up around 700N.
I’d also aim for 900N on the front wheel.
All tensions assuming zero tire pressure.
Considering the low spoke count and the high tire pressure you are using you could go for even higher (initial) spoke tension.
I’ve had good experience with using grease on the spoke nipples (on contact surfaces and thread). Makes truing much easier, avoids wear from truing and also avoids deforming the spokes or nipples from all the tension force.