in after effects, a simple expression for the width and height to fit in a precomp so any picture would perfectly size and the precomp would make it all reusable and a lot faster. I don’t think premiere was made to do this sort of stuff automatically. select clips, right click set to frame/scale to frame.
maybe a nested sequence might work, I seriously doubt it. Sounds like a lot of trouble in premiere though. I recommend AE.