…or are you using another platform to create set extensions after the fact….
I always keep my sequences in camera native (or % off in case of 8K Venice) resolution and use letterboxes. I then append the aspect-ratio to the clip name (e.g. 9x16) .. then I just grab everything and my export-script scans the clip name for the aspect and does a crop edges (basically it selects a matching export preset) upon export. Saves me a lot of headaches. The one thing that I want to fix in my script is that when overwriting I have to hit overwrite every time as it submits a job per clip..
I considered something like that, but felt it created as many new headaches as it solved. For one thing, there is so much repo and pan/scan in the narrower aspects that each needed it’s own timeline anyway, many jobs have multiple cameras, so I would need to decide on a “default” resolution, and I get most of my graphics from other artists built to the specific delivery spec which is almost always smaller than the native camera resolution. In the long run I feel more comfortable knowing I will be exporting exactly what I’ve been seeing as I construct each spot.