Reading the manual and some of the posts here, I wonder if Outline could sync to a Windows Webdav server running on my lokal Windows 10 installation. I don't want Owncloud/Nextcloud but would like to sync without the use of cloudservices.
Before buying the Webdav feature in Outline I wanted to be sure that it will work.
Another way would be for Outline to support cryptomator, I can open part of OnNote notebooks with Outline from the iOS Cryptomator app, but I can't copy them back. Maybe you could integrate cryptomator like a you have done with other cloud services or allow to save into the new file app in iOS.