Tbf, I'm not good enough to not restart a mission whenever I lose a soldier. But I have learned it pays well to sell nearly everything (except for the things needed for buying weapons) early on to buy better weapons, as well as Vulture and Wet Works.
I have been holding off on Proving Ground stuff for now, waiting until a region is liberated so I can get an income. Not sure if that'll pay off like I'm hoping tho lol
(LWOTC)