Welp, I'm right back where I started, but I did learn a few things. Attaching some photos for reference.
The owners manual shows a single return spring on the trigger linkage, whereas mine has a two-spring setup, so Nosler may be making modifications to their design over time.
This is "stage 1" of the two-stage trigger, and it seems to be ~1.5 lbs. The trigger travel/return in this stage is relatively smooth when the action is removed, but it gets worse after reassembly, so I'm assuming there is interference somewhere.
I also measured these two springs, and they appear to be the same as the one in the sear mechanism that I measured in the previous post above.
Now onto the sear mechanism itself. I tried several lighter springs, but the effect on trigger pull weight was negligible. Therefore, I believe the second stage "weight" is largely dependent on the sear engagement (i.e. friction). Lightening the spring only made the trigger reset less reliable.
The sear engagement screw, however, appears to have a coarse thread, so the adjustment is not as precise as I was hoping for.
After spending 3 hours tinkering today, I reverted back to the original spring, and the two-stage trigger is still 3.5 lbs (i.e. ~1.5 lbs in the first stage and ~2 lbs in the second stage).
I really wish the first stage was smoother. Now I have to decide if it's worth sending to a professional for improvement…
I'm open to ideas/suggestions.