On Desktop Safari the reading list is stored in a Bookmarks plist file you can grab and filter out Reading List items from, might be worth having some little desktop util for a 1 time sync to move people onto your platform? It's not ideal, but it's better than nothing.

Here is an example with some code showing how someone did it so they could upload it to Pinboard: https://brettterpstra.com/2015/01/06/reading-list-catcher/