First time buying proper dress shoes. For some background, I have very flat feet, which I suspect is part of the problem. I tried on some oxfords at an Allen Edmonds store and found the 8EEE most comfortable, though I think the Brannock device said I was 8EE. With the 8EEE, there wasn't really any slipping, and going any narrower felt uncomfortable around the mid foot and balls of my feet.

I just got a pair of vintage Park Avenues on eBay (so I wasn't able to try them on) in the same size, but they feel quite loose around the ankle/heel area. I suspect the other person had higher arches or thicker feet that caused the back of the shoe to raise up over time. When I insert an orthotic, it reduces the looseness a good bit, but there's still some I show in the video. What I want to ask is, how should I address this? Should I size down to an 8EE and hope the leather expands a bit over time? Should I add more padding to the heel or tongue to get it to sit more snugly? Something else? I got this particular pair partly to confirm the sizing because it was pretty cheap ($60), so it's not devastating if I just need to go down a size in width.

Pictures and video https://imgur.com/a/ZHGHSu9