Grades only compare you to others on the same champion in the same role, so orianna support only needs as much cs as other orianna supports tend to be getting, and doesn't require cs numbers anything like what orianna mids are getting.
It's unclear to what degree different builds of the same champ in the same role impact grade requirements though. I doubt they differentiate them grading wise, and it's just in theory what one build may be better for in cs, it loses out on in kda, or vice versa, and since grades take in loads of factors (notably not just cs/kda), if two builds are equally viable, they'd both be able to produce comparable grades.
I'm not sold that it's necessarily a lack of cs that is holding grade back. Improving cs never hurts of course, but theres nothing in your grade that outright says 'you need more cs', so it may well be that your cs is good enough and you just need more objective participation/kill participation/vision score/even better kdas/etc.