if not stacking while only lasting 8s max, that serioulsy cut its usefullness, forgot about the grace requirement you put indeed.
so if i get it right, in the "best" possible situation, this would turn a 100% overhealing renew in 60% (before mastery) of the same amount in multiple shields (i'll let aegis for stacking shields). and for that shield to be effective ofc, that would require the shielded player to get hit for enough damage.
on one hand, if renew is meant to overheal, this may turn it into mini shields (sounds a bit like a weak remake of earth shield in the end except for the effectively "healed" amount which is... totally unpredictible) but then wouldn't it have been better to throw up a mere pw:s that cost if not less about the same mana and shields for more? well, i guess this may help a bit while weakened soul is still up...
on the other hand, without overheal, it won't do anything more than the passives : 10% reduced mana cost & 3s increased hot time which is still nice but won't make sure renew is a better spell for disc.