I don't have my ipad handy to test with, but I'm about 99% sure that the photos folder isn't user exposed in the Files.app. iOS 13 did _finally_ add USB Mass Storage support, so one thing to test would be to get a lightning>USB-A camera adapter (if you get one with the included lightning power pass-through, I think it's USB3 speeds), or a USB-C > USB-A adapter and connect your external and then in Split View, have your photos open on one side, and Files.app open in the other. Drag the photos from Photos.app to your external.
iOS is still pretty limited on that sort of front though, there's no conventional progress bar (that I have seen), so when I was backing up lots of large files from my iPad to an external, it wasn't easy to know for sure how far along in the process it was. I'm also not sure how well it would handle being backgrounded (I'm sure that it would be fine, but iOS 13 has had lots of little issues from things being not quite ready for primetime).