Yep, both agree here. A sidecar/backstory also does way better justice to the character than “flashback short”. Take Jeff, briefly mentioned in a flashback, is now a fully fleshed character, and the reader will be rooting for him when he competes in the Olympics (swimming), in what’s likely going to be a multi-installment type of back story.
One thing I fight in my potter story rewrite… I did a lot of retcon before, so it’d be like Hermione going “Yesterday Snape talked to me...” On this rewrite, I’m changing it to showing that conversation when it happens. Thus, what had been revealed later to be SeamusFinnigan/LadyAuror relation, is now being shown (added a new character on this rewrite, and his POV is able to witness SeamusFinnigan/LadyAuror banging on the sofa in the Gryffindor Common Room).