Nominally, the STAG-10 is an AR-10 platform designed by Stag Arms. I want to replace the stock trigger with an after-market drop-in trigger with a pull weight less than 2.6lbs.  Is this possible?

The reason I am asking this is because I have heard rumors that STAG-10s have proprietary lowers which only fit into in-house Stag Arms uppers. Ideally I would like a drop-in trigger if possible, but I'm willing to try more complicated replacements like Geisseles.  Your thoughts?

According to the STAG ARMS site the Stag-10 lower can use Mil-Spec fire control group.

If so, you can get a drop-in like the Rise Armament 434 or 535,  Trigger Tech Diamond drop-in or Geissele if you wish.

from STAG ARMS site

https://www.stagarms.com/stag-fire-arm-parts/receivers/lower-receiver/stag-10-stripped-lower-receivers-bla-stag300928.html

"The Stag Stripped Lower accepts any Mil-Spec Trigger Guard, Mil-Spec Fire Control Group, Mil-Spec Stock, DPMS Magazines, and Standard AR15 receiver extension tubes with a .308 Buffer and Buffer Spring. Accepts any standard AR Pistol Grip."