First, leave tranquility alone. its mostly fine the way it is, maybe lower the CD to 5m so we can use it every attempt.
Then make the .5 second rejuv talent baseline and replace it with [Freya's Gift] - it turns our 3 mushrooms into a single big mushroom (like the ones on Freya) that provide a 15-20% damage mitigation for anyone who is standing under it. Not only does this solve the problem of druids not having a mitigation CD, but it also fits with the current, "make everything as inefficient as possible", design philosophy for druids.
If they wanted to not get so fancy with the Freya 'shroom they could just have a green AMZ zone (with flowers and shit floating around in it, obviously) form when the mushrooms are detonated. Maybe make each mushroom a 10% mitigation buff. Obviously the CD on detonate would have to be increased to 2-3m if this talent was taken.