Not sure if anyone cares anymore or if this will ever help someone who stumbles upon this thread in the future figure out what the issue is, but I have finally run across a game( b) type from the post above) that works properly.
Here’s a video with it. As you can see no black bars, no elements getting cut of, etc. just perfect execution!
This is how Songs Of Silence is supposed to work as well and what gamescope should try to emulate/achieve when you run a game with it that doesn’t natively support resizing like that.