(for instance, the dude who becomes a full member by only shit-posting for 3 month period doesn't deserve to hold that high rank)
I see your opinion here and in fact not only full members are got ranked because of shitposting. There are other higher ranks too that have obtained their current rank now by just shitposting.
I know that's kinda hard task, but it can be done either by adding some requirement for obtaining "monthly merit point" (for instance 1 point for Junior 2 for Member 4 for Full and so on) thus shitposters won't be able to get away that easy.
Can you share some requirements you are thinking for this "monthly merit point" you are saying. I think this is a good idea but I don't think there will be no other way to gain some merit right now.