i would like to know if he just has the same ending regardless of decisions made throughout the story (excluding the endgame decisions like ascending) or if it's different depending on what happens during his personal quest
I let her live and get the dragonling. I'm sure the ending will be different as I assume the game triggers what ending narration you get based on the quest journal After all, if Sadha is dead, he can't be a dragon daddy as mentioned in the ending.
I actually considered going into the game and secretly kill Sadha before going for the last boss but considering how he dreams for so long to have dragon children, I rather have my char commit suicide in secret after they say goodbye than to have him suffer in the name of love. Too painful to go and try it out so someone else will have to do it.