There is also the possibility of splitting her work and sharing it across both of the next two movies so she has a presence in the whole trilogy. as we found with Rogue One films are often restructured after shoots and there is a tendency to film more than finally needed. I would just film more stuff with Mark and the new peeps and allow the Leia footage to bridge both films.
The main reason I haven’t looked at things like that is on the question of reshoots. I heard a rumour (Though I don’t know whether to believe it’s true or false, but I want to say false) that Rian Johnson didn’t actually schedule a lot of reshooting time for VIII, limiting the amount of new material that could be made from a reshoot.
If so there may only be so much that can be done to rectify in VIII her absence in IX’s shooting.