I have every intentions of doing all missions at some point. However, right now I'd like to focus on "important" missions. Not sure if the important missions list is longer than the non-important. So with a primary focus on completing missions that allow for sellable rewards (planet upgrades or transformations, limited artifacts worth stealing a planet for, etc), can anyone suggest what missions should be completed?

i cant tell you what you want and it comes down to you reading mission chains and deciding what you want

mylarai chain >> hull upgrade
kronyn chain >> ergosphere filter
for everything else theres the wiki