I don't think he'd be very good at it tbh.
Usually, starting pitchers who turn into good relievers are guys that have an elite pitch that they can over rely on in one inning stints.
Manoah doesn't have a pitch like that. Even in his 2022 season his slider got a 118 Stuff+ score. Since then, even that pitch has been below average along with all of his others.
He was always more of a kitchen sink guy which helps him as a starter.