I think the combination of nature studies and overgrowth and wild growth is a bit extreme. But Animal Guardians is almost like a 7 mana Ultimate Infestation. Often you draw 2, you get a ton of stats on the board, you often deal a lot of damage to the enemy.
I think you could honestly take out the rush aspect of the card completely.
Lightning Bloom isn't honestly that broken. It's more the existence of Overflow and the synergy with stuff like Exotic Mountseller and other effects that makes it a problem. Also Animal Guardians is just a bonkers strong card in druid with what they get access to. It's also that being able to get so much tempo and board impact in a class that is balanced around not having board impact is so powerful. And the fact that Innervate is still a card too. Meaning you can make 7 mana plays without having to even invest much.
I sort of wish there was a way for Kael to be more balanced across the board. I don't think the 1 mana change would be too bad. It might make him completely unplayable, though outside of degenerate combos, and he was a gift card so we don't even get compensated for the nerf. "Come back and play AoO! Get this free card we keep making worse!"