It could alternatively possibly just have its charge per use upped to two like Elephant's Hide. This would allow characters to simply now have to choose which of their WS based abilities they want to prioritize per rest.
Quick edit to explain my thinking here:
In my experience, for a high level druid (the ones with which most of the OP concern rests) elemental essence does not last the entire rest cycle. So if you cannot finish your dungeon before all of your regular wards fade, you must use it twice. If the cost was higher, a druid with all four feats would have to choose if they want to have 2d6 damage consistently, or have 4d6 in the beginning and risk finishing out with no bonus damage at all for the equivalent amount of WS used. Additionally, bow druids (which seems to be weaker in the concensus of this thread's posts) would be less penalized by higher charge price than melee druids, since they don't have as much reliance on the defensive WS feats.