First Lite. Hunting company. I've worn their Merino boxers for the last 5 years, no major complaints.

I run the exo mountain gear packs for hunting, can easily run 100+ in them.

https://exomtngear.com/

I also used the USMC main pack (FILBE) and humped 100+ pounds in it for 3 weeks at mountain warfare training center in the winter.

One of the biggest issues with weight is not suing the hip belts correctly. Should ride high on your hips and extend above your head.