Now, this may seem presumptuous, but I feel like that’s one of the easier things possible. Once he decides to stay with Jannah on the Destroyer, we get a montage of hologram message playing on FO Destroyers, using AI voice lines and his model from Battlefront with a hologram filter over it.
I didn’t say it would be good, but a mockup should be possible. I’m tempted.
Then it sounds like this would be our only option for the idea.
There is a deleted scene of Kylo Ren on a Star Destroyer bridge. He could be VFX’d out and replaced with the hologram.