Once I'm level 400, how do I get rich yields for 49-50?
Is there any way to add new missions or anything?
The only "rich" grade 6 missions are companion gift, but even those are technically Rich grade 5.
Some crew skills have a fixed set of five missions at grade 6 (level 49-50) like archeology and slicing. Some are more variable but you will never see a materials mission greater than abundant like Underworld Trading.