Hey, the topic of how share ProseMirror steps with a Python backend has been a few times in the past. In our current version 3.8 of Fidus Writer, we are sending changes to the server both as steps (for redistribution to other clients) and as a RFC6902 json patch for the server to apply to the document that is to be saved. That is working, but obviously it brings a number of issues with it.
So for the next version I have been looking at using https://github.com/fellowinsights/prosemirror-py directly on the server. That works fine in testing and also the one major hurdle there was (loading the schema) was not that difficult to get around.