I would expect that reporting them doesn't do all that much, since Instagram doesn't really have an incentive to make that right for you (unless, of course, you're a large content creator with a ton of followers; in that case, I'm sure Instagram does want to keep you happy).
Instead, file a DMCA takedown notice with Instagram referencing the stolen work. They're required by law to comply and take it down. I expect that most content thieves won't see it as worth it to contest the takedown and open themselves up to legal liability.
Not true at all, I've reported MANY of my unauthorized posts and all have been swiftly dealt with by Instagram. In my experience, they take it seriously when you provide proof of your post, and theirs.
Instead, file a DMCA takedown notice with Instagram referencing the stolen work. They're required by law to comply and take it down. I expect that most content thieves won't see it as worth it to contest the takedown and open themselves up to legal liability.