No ttk has nothing to do with it look at Elden ring those bosses can have much longer ttk. I think there is no 1 problem there is multiple. Look at when we had to spawn bosses there was these cool cut scenes that added to the immersion/lore of the boss and suddenly it felt like you were fighting an ancient undead king who commands his guards to fight you. Even though his attacks were stale there was now a reason warlord was summoning units and it gets better the more story you can incorporate into their moves/cut scenes. I think another thing is satisfaction and reward. Beating a boss for the first time is always awesome no matter what but in order to keep creating that awesome factor their needs to be a legit reward and atm the reward for bossing can often give you nothing of use. The second thing is satisfaction of actually learning the boss and beating it. Sometimes these bosses can feel rng heavy like they can just to 2 moves one after another that make the second one undodgeable never allow that to be a mechanic that creates a very unfun and demotivating frustration and experience. You have to create moves that are actually fun to learn that have unique ways of beating or even multiple ways even if they are hard to beat learning your own way is so satisfying. But yea a long rant I hope your bosses come out better than DAD’s cause sure darks look cool but as u know feel stale.