Suggestion for Soraka's heal

Soraka's heal has been a problem since she exists. The problem lies in the search for the perfect balance between lane sustain and relevancy in team fights. If she's relevant in team fights, her heal would be too powerful for laning and a heal adapted to the laning phase would fall off too quickly and become irrelevant.

To fix the issue, I've thought of using a mechanic similar to Kassadin's utimate: successive casts, bigger mana cost, better effect.

Every time Soraka uses Astral Infusion, subsequent casts cost 100% more mana and increase the heal by 25%, stacking up to 4 times. (stacks disappear after several (10-15) seconds without casting Astral Infusion)

This way, she can't spam her heal in lane because It would be too expensive in mana, and it still allows bursters to kill squishies before Soraka can stack. However, in extended fight, she can become a real pain if allowed to stack and will feel useful, thought it would still cost her 40% of her health to get a fully stacked heal.