Custom Aspect Ratio for proxies

It's a bit of a process, sadly. You go to MediaEncoder, make an endoding preset. Then you make an Ingest preset, that uses that encoding preset inside it. Store it to the right folder on you rig, relaunch Premiere.

While the information on how to do this is in the app help, I'd suggest watching tutorials by Karl Soule or "Video Revealed" on YouTube. Karl's one of the lead staffers in the Premiere "Hollywood" division, and VideoRevealed is the website of a very experienced editor. Paul Murhpy's "The PremierePro" has good stuff also.

Essentially, you make that encoding preset at specific fractions of your framesize. Typically a quarter or half for each dimension. And using an intraframe codec like ProRes, probably ProResLT, does allow for fast playback.

By selecting ... or creating ... a proxy preset that matches your original clips.

By "full frame", what do you mean? Different cameras have different aspect ratios to their sensors. And video work ain't stills ... which I learned the hard way, coming into video from a long career as a stills pro.