Vader's arrival over Scarif inadvertently explains in-universe why we don't see the Devastator after the beginning of A New Hope - it was probably at the shipyards getting patched up after taking a rebel ship to the bow
While there's the obvious out-of-universe explanation that Lucas hadn't established the Devastator as Vader's flagship at the time, and the Death Star was the main focus of ANH, the collision is a pretty convenient explanation for why we don't see it after the beginning of the film. Once they'd successfully captured Leia, it would make sense for Vader to dismiss the Devastator to go get patched up at Kuat Drive Yards or the other nearest shipyard. Vader doesn't strike me as the type that likes his personal ships looking anything less than perfect.