This deck is a ton of fun to play, and surprisingly powerful.
You want to mulligan for an Elrond's Council or Resourceful. You want to stay in secrecy for as long as possible. Against Nightmare Anduin, I was able to remain in secrecy for the entire game, so it should be no problem in most cases. I've beaten 4 different nightmare decks so far with this deck solo (and no loses), so I feel the deck is very strong.
The deck's main strengths are it's ability to quest, but it also has lots of attack power once the Ents get going. If I make any changes in the future, it might be to remove some resource generation in order to add more card draw, as in all four games I've played so far I just stopped adding resources to Galadriel because she had more than I'd be able to spend in three games.
Besides Steward and Ethir Swordsman, the deck FEELS very thematic. I wanted to go with a Galadriel theme, hence the Keen as Lances instead of Gandalfs (and I just LOVE that art). But if I was going to optimize the deck, I'd remove them in favor of Core Gandalf.
Victory display shennagians are fun, and this deck does it well. Hope you enjoy!