I think the Mustafar Minute could be set up by moving the Vader helmet scene to the very end of TLJ.
Kylo had just been tricked by Luke’s force projection, the Resistance escaped, and Rey shut the door of the Falcon on him, so he goes to Vader’s mask for some guidance - which is something he’s been doing since TFA.
When he touches the mask he sees Mustafar (use RO footage or other lava related footage), a brief snippet of a wayfinder and Exegol, throw in a Vader “join me” and a Palps “you will go to the Mustafar system” voiceover,and end the movie.