Seraph and Eternal are armor grades. So the R110 armor is Leviathan and not Krishna (this is for weapons).
The S can count for Super like you said, or Special. And the R, just simple a new way of grading things after GoD.
Same way that Bronze, Silver, Gold wasn't enough to define ranks in some games, and they added Diamond, Platinum and so on. It doesn't need to have a meaning, just that they need to keep upgrading and they need to find new rank/grades for it. They might run the whole alphabet one day
L2 Lore can be found here, but no info about why recent things are named as they are: https://l2wiki.com/Lineage_2_Lore