This might not be the best answer and is most likely incorrect, but often times I don’t include “transition words” necessarily.
Sometimes when I start a conclusion paragraph with a transition phrase, I find myself repeating word-for-word sentences I already used (like topic sentences from body paragraphs) instead of actually writing something that brings the essay together. Instead of writing “in conclusion, the theme of the story is _ because x, y, and z” I would put something like “xyz shows how the theme of _ is prevalent in _. The characters face challenges like x, that follow them through the story into yz.”
I hope this made sense. I think transition words can make it harder for an essay to “flow” and sound cookie-cutter-ish in my opinion. But I’m not super knowledgeable at writing essays so I don’t know how helpful my advice will be.