This sounds like a perfect application for EROFS[1]. While it comes from an embedded systems background, it has seen some usage in container use cases and is moving towards a general "mountable tar" application. It would also avoid the tedium you have to go through in shrink_btrfs.py because you can just generate the image out of a tree.
I wanted to give repackaging the btrfs image a shot but the download was pretty slow - I assume your server is getting HN-hugged a bit so I didn't want to make it worse and stopped the download.