That’s awesome. I actually got it working last night but woke to see this and it’s a great explanation and clarifies what’s going on so thanks a lot!
Presumably, to make sure game positions/vectors don’t get too confusing it probably makes sense to get the whole game working correctly at the small resolution and then simply add in the scaling code?
Also, does it help to make sure you keep things in multiples of 720 and 1080?
Seriously great idea and saves me having to doing sprite atlases of correctly rotated pixel art!