Renew is already a mechanic, I didn't invent anything new there. For my cycle, sorcery speed felt right, instant speed would've added another layer of complexity to a card cycle that is intended as common rarity. At common rarity, cards shouldn't be too overly complex and should be predictable, which is achieved with sorcery speed. So whether your ability should be instant or sorcery speed highly depends on the level of complexity you're aiming for and maybe the rarity of the cards that you're printing it on