I agree with you both that it could be jarring to suddenly have Barriss appear and take on a major story role right at the end. I have thought about combining it with Ahsoka’s final episode in Tales of the Jedi. It would make a great episode in a TV show format but not as a feature. I think I’ve managed to produce something that works as an epilogue to the movie.

I have put the first Barriss scene during the order 66 sequence, so she doesn’t appear from nowhere and I will mention her in the opening crawl. In terms of the rest of the Barriss content.
The Siege of Mandalore arc already has an epilogue of sorts with Vader finding Ahsoka’s lightsaber. What I’ve done is use that scene, a condensed version of Barriss’ recruitment to the Inquisitorius and the Padme funeral scene to create the epilogue. It plays out in chronological order with small time skips and shows us what directions these characters are going in as we head into the Dark Times and also hammers home that dark times are ahead.

Here is the sequence. Let me know what you think.

https://drive.google.com/file/d/1OaSO65TfzCiXSPZDmHsTI78g8X90877o