Maybe this is silly, but once you've unlocked the story (for prisms) and the costume (if you want it), is there any point in leveling or keeping R-level cards? I have enough different SR to fill out my teams so what's the best use for all these Rs?
Special practicing and max levelling any card yields 20 prisms, so if you can at least do it with N’s and R’s its definitely worth it. Past doing that and getting the memorial boards/moments if you want to, it really depends on each player!
I have always liked to keep a collection of all the rare cards in the game max levelled and idolised etc until I run out of space to do so. If you want to do the same then do, but otherwise selling R’s once you’ve unlocked everything for them can also be beneficial. It really depends what you want to do! You can also use them to fill teams and build bond with your best boy.