So what might have been Elan's super secret plan?

[Peelee]

SW is tricky though, because technically any and every series/film set prior to the end point in RoS now counts as a prequel.

What? That's not right. The original movie is the point where sequel/prequel delineation is.

So Mandalorian is a prequel

You are the only person I've ever heard that from. And it's wrong. Not only because it doesn't take place after the original movie (except for small portions in flashbacks), but also because it's not following the same characters or story. Rogue One and Episodes I-III are prequels because they still follow the same characters or story, just from earlier. Mandalorian is an entirely separate story that also takes place later. It's not even a sequel. It's just another story in that franchise.