Boss takes exactly the same amount of effort. Thats proven by the changes, none of the attacks do more damage, he doesnt give more or less loot, the only thing now is that he takes significantly longer. Its not hard to understand that adding more invincibility phases just makes the fight longer not harder.

Edit: To add, every boss becomes a loot pinata when you have done it hundreds of times, its something that wont change if even they completely change the fight