My solution is basically what @mojolobo mentions with Nextcloud behind it and I love the concept. Because Obsidian (via a WebDAV plugin on the phone) just syncs with the “Notes” folder in my Nextcloud root it really is just a bunch of .md (markdown) files. It gives me an added sense of security (on top of the self-hosting aspect) because I can see those files everywhere I have Nextcloud installed, I can edit them manually if I wanted to. On the PC you just point the Obsidian app to the folder, on phones you do it via a WebDAV plugin.