Overcast produces an export with everything, but warns they don't know of any apps that import it. The nice thing about OPML though is it's XML. An importer will just skip what it doesn't understand and import the subscriptions.
Podcast Addict does that, you can backup the full app settings, and more than just the subscriptions/listened episodes. I'm guessing the format is specific to the app's export/import feature, but could be worked with.
