Here’s an idea: what if Kylo builds his helmet on Mustafar?

Opening: exterior of a First Order Ship
Cut to: Vader’s mask container opening and Kylo touching it to receive a vision. At some point during the vision, Palpatine’s “Mustafar” from ep III can be heard. End with Dominic Cobb’s idea of “you’re a creature in a mask” “yes I am.”
Transition to: Mustafar from space, the original TROS opening.
Cut to: edited shot of Vader’s castle from Rogue One.
Cut to: the pieces of Kylo’s helmet being dumped onto a table.

From here, the helmet reassembly and Mustafar massacre scenes cut back and forth to each other as a montage. This leaves us with two potential ways to end the scene:

Helmet is fixed, Kylo picks up the Wayfinder, and the original TROS transition of the wayfinder to Kylo flying to Exegol.
or
Kylo picks up the Wayfinder, Helmet is fixed, and the scene ends on the shot of the Helmet. This allows you to move to a different scene if you want and push the Kylo’s trip to Exogol (and thus the Emperor reveal) a bit later into the movie.

I feel this plan uses the fast-paced nature of the Mustafar sequence to our advantage, instead of trying to just drag it out into a full scene in of itself, while also adding even more weight to Kylo’s rebuilding of the helmet, since it’s now a ritual [implicitly] taking place within Vader’s own castle.

The only issue I can see, assuming the Rogue One shot can be properly edited, is that Kylo’s trip to Exogol still occurs without the helmet. But maybe people could forgive that since it’s such an important meeting?