I don’t think “scavenger” being the last line works as well as a back-and-forth, though.
If Palpatine’s last line is “I am all the Sith” and Rey says “and I am all the Jedi,” then she’s following up his boast with her own (corny as it may be), and if his last line is “You are nothing” and she says “no, I am a Jedi,” then she’s refuting him and asserting herself. But if Palpatine says “a scavenger girl is not match for me” and Rey’s direct response is “and I am a Jedi,” it sounds odd, almost like they’re having two different conversations.
That’s why I think trying to get “nothing” in as the last line would be worth more investigation, because it also has that personal touch for Rey, arguably moreso. She’s always known she’s a scavenger (and she doesn’t get emotional when Poe brings it up in the same movie), but being called “nothing” by a dark sider specifically touches on her emotional low-point from the last movie and her core insecurity.