casting fog or darkness on the act 2 end boss probably makes it easier if some of the party has immunity

Exactly what I've been thinking! I really hope it doesn't have some hidden immunity to it or it could end up a sketchy fight, especially when it has legendary actions.

Druids also get create water, guidance, and Heroes Feast, so maybe you're not missing out much apart from a dedicated healer. Also, they can get Water Myrmidons; the best summon imo. My plan for reverberation is to get a Circle of the Stars Druid for radiant damage on bonus action.