Is there a way to have dependencies between images? So I could have the python-image build know which dependencies are already built into my OS image and therefore not include them itself?
No. An image is a self-contained file system. About 50% of your image recipe isn't helping your goal of an image that contains a single package.
I have to query your assumptions though: if you're going to have python3 in a filessytem that you mount when required, how are you saving space?
Ross