The legendary action Create Blights should be called something else to minimize confusion, and probably should either just trigger without having to roll a d6 or only take 1 legendary action. But this is really cool, I remember when I played Sunless Citadel the party was confused when the DM insisted that we were still in initiative order but the tree didn't do anything to react while we chopped away at it. 

the thing is, that the fight itself is already a big one for level 2 people. adding the tree  really adds to the challenge and makes the fight much harder.
even more if the tree just summons  a lot of stuff. characters should be level 2 by the time they get to the tree. but... honestly, my players have a really hard time in the citadel as it is so imagine if i start making encounters even harder.

though i like the idea of a tree with abilities, we're definitely in need of more plant monsters.
but thing is... if the tree fights back and helps those creatures the tree created. you're changing the hardness of the challenge and make it much more challenging.
we also have to consider that the tree can also absorbs the players and change them into villain versions of themsevles under the trees will. that can also be a thing.

The adventure synopsis recommends that the party be 3rd level before that fight:

They should advance through 2nd level to 3rd level before the finale. 

level 3 seems right, but still... by adding damage dealing abilities to the tree, you still add more CR to the thing, thus making it harder then it is. and its already hard as is.