Is there a shot of that cask with the doors fully closed before they start opening?
I wonder if, after the tilt down where the stars disappear but before the planet & ships come into view, you could have the doors open on the mask. It would need some nimble music editing, but I think you might be able to go straight from the tilt down to that thing opening up with a pretty clever cut and some brightness/contrast adjusting on the surface of the cask itself.
I also wonder if, opening straight on Vader’s burnt mask, you could combine parts of the Mustafar minute with his visions (it would need more cutting back and forth, and decreasing the amount of time spent watching him touching the mask - split those shots up into smaller cutaways, basically) and his taking his hand away and whirling out of the room would then cut straight to him flying to Exegol.
Maybe that’s the only real way to make the Mustafar Minute make sense without having to create establishing shots in VFX - build it into visions/flashbacks, so its disjointed nature is absolutely presented as intentional, and not a result of bad editing making the sequence incomprehensible.
If the combined vision/mustafar killing spree ends on him grabbing the wayfinder and cuts back to him taking his hand off the mask and leaving the room, it should work.