I went through the game on playthrough 1 on hardcore, never upgrading. I then switched to casual and the rest of the points and journals. After I had enough points I upgraded each weapon and the the jetpack at the box in 2-4. I too also only upgraded the 1st slot weapon and the swapped it out for the next weapon. I did this on casual and never switched back to hardcore. I also stayed online. Worked great.
-Insert something creative here-
( that doesn't include stats that no one really cares about or some poorly photoshop'd image with your name stuck in it. )