Getting those shards is imperative right now in order to get a 4th character and access to stella strike, also it seems most of the R equipment is out of reach for the f2p characters only lapis can equip r equipment thanks to her high int, rubi can only equip an R leather knife , the pickaxe, necklace and fighter armor, linette can only equip R earrings.
We're currently in the middle of the abyss invader event so its a good time to stockpile on those STFY rewards that we can use to purchase additional R shards if needed, plus we can use the medals to buy gemstones for the R neck/earrings however this would be long term for the reasons previously mentioned, a better use would be on the potions that allow us to clear a stage instantly, I think I'll wait a bit before spending the medals.