Local dry-run pruning looks good for the overworld. I'm also happy to go with the new seed suggested above post-upgrade. If nothing else, that saves me a lot of trouble experimenting with seeds and image overlays in roughly the same way. So, thanks for that!
Here's what the current regions for the end and nether look like.
709 (or anyone else), you want to take a stab at marking off some chunks to prune there?
Maybe just keep the center 2x2 regions for both? That preserves the nether hub hallways and the end center island.