Bake new UV Image With old UV as Clone Source

I’m using this trick where you unwrap your mesh one way, for convenience, then you create a second UV map, unwrapping the mesh a more correct way. Then you use the Texture paint/clone tool to move the color data from the first UV to the second.

The images i’ve used for the first UV unwrapping are very high res, 3-6,000px. I created a 12,000x12,000 image to use as the base for the second UV texture map, onto which the color data from the first UV will be painted onto.

If i bake, i get a black border around my UV layouts, but not the color data from the clone source. Am i doing something wrong or is this not possible? Since it’s so high-res, painting/cloning brush really kills my computer’s processing capabilities, and it does crash if i go too fast. Baking would be far more convenient.