That might work. I recently watched an edit that moves Palpatine’s reveal to the end of act 1, and while the cutaway kind of helps the “Scavenger Hunt” plot feel less hectic, it makes seeing Palpatine less shocking because you’ve had more time to settle into the rest of the story.
I’ve experimented with putting Palpatine’s message into the Last Jedi while saving his visual reveal for tRoS, but the only spot I can find is after the credits. I would hate for tLJ to spend 80% of its runtime being so bleak, and just when you think it’s going to end on a positive note, it turns out the big bad has returned from the dead with a promise of revenge. That feels cruel.
You have a point about ending on a hopeful note, and while that wouldn’t work with Palpatine’s message, it would absolutely work with Kylo’s trip to Exegol. It’s something the protagonists don’t know about, and it shows that the antagonist has been weakened to the point of seeking outside help.