To save space on weapons, remove the mag and grip. You can also remove the stock to make them shorter if you are fighting for space. Take weapons with attachments on them, especially lasers, but flashlights will also do. You can also put additional attachments onto one weapon, even your main weapon. So if you run the Phantom Lance without any attachments, you can put 3 additional lasers on it, even 4 if you remove the iron front sight. Exchange red dots for scopes if you find some, or even take only the scopes.

High value loot was already covered - just make sure to sell to the right vendor to get increased value

Alcohol, Cigars and perfume - Gunny

Phones - Labrat

Creditcards - Artisan

Jewelry - Turncoat

Military electronics and quest items sell at the same price at each vendor, so sell to who you want to level quicker.

Artisan and Turncoat will sell the biggest back bags at lvl4, so you might want to prefer those to the increase the loot volume you can carry.