I was able to find the scaling value for the GUI after some work. However, it is currently unusable because it's constantly being rewritten (re-calculated) by something else, so this solution is not viable at the moment sadly.
It is constantly flickering in-game like this because Cheat Engine isn't fast enough to freeze the value how I would like it to be.
Due to this, this hack is currently unusable to play through properly.
This is a still frame where the GUI is correct at 1440p native. (No AA again for pixel-counting, click for full res)
So yeah, it should technically be possible to somehow fix that with a dirty assembly patch or tracing back the source of this value even further. So I'm just gonna wait a few weeks first to see if Platinum decide to fix the resolution thing themselves. If they do abandon the possibility of that tho, a resolution fix should be possible with just memory editing alone.
Honestly what bugs me the most so far is the refresh rate workaround. That 1920x1079 fake resolution makes everything look blurry, and I like the sharpness of native. : / At the moment I'm resorting to the nice Cheat table someone posted here that forces D3D to use 60 hz instead, so I can play properly now. It just gets a bit annoying to have to boot Cheat Engine along with it. So the buffer thing is not a priority IMO, and it would actually require more work due to having to rework how the GUI scaling is handled. (Although a proper implementation would kill two birds with one shot)