Hi Mark,
There's very little discussion on the mailing list regarding migrating away from gallery 3. It's almost like walking into a Mercedes mechanic and announcing that you are trading your car in for a Toyota and then asking the sales manager for what Lexus or Toyota he recommends.
In the interest of not responding in kind I will say that python Django is almost entirely bespoke and uses mostly SQLite, though postgres and other drivers exist. You're not going to be able to port the database over.
I archived over 3000 images from a gallery that was hosted in the mid-2000s from
archive.org and literally the entire process was manual. If you're working with such a python framework then you probably have a developer on staff or rather should, and if you don't I'm going to tell you that in the future working with python crap you're going to be running into trouble. PHP has been around for over 20 years and while it's not the fastest or greatest it's widely understood and you can find inexpensive developers looking to help you with it. The same is true for python to a lesser degree but you also have a lot of these new developers from other countries that really don't even know how to migrate from other things into python.
Bottom line is I wish you the best with whatever you want to do but you're probably not going to get much help for it here and I would strongly recommend asking elsewhere for assistance.