ObsidianMD

Syncthing for me https://github.com/Catfriend1/syncthing-android

Exporting the .md file. I also think that for fellow Obsidian users, the bits that have functionality in Obsidian that don't in Markdown in general might come through if they open the file in Obsidian, though I'm not 100% sure.

I know there is probably a plugin for this, as well as the official Obsidian Sync.

https://help.obsidian.md/getting-started/sync-your-notes-across-devices

Out of the options listed here, Syncthing seems to have compatibility with all of your things.

Edit: sorry, I've realised after writing this that I misread and the post was about hosting for sharing, not for backup and sharing between your own devices. I'll leave my comment up in case it's interesting anyway, but I wrote it thinking this was about having your vault synced across devices.

I've used a few things for both Logseq and Obsidian, Google Drive with the desktop app and FolderSync for android was the second best I've tried.

The best I've tried is Obsidian sync, so the few quid are worth it for me as it saves me a lot of hassle and I would say Obsidian is one of my top 3 used apps.

Maybe take a look at the Remotely Save plugin for Obsidian. It works with lots of services (though not sure about Proton). The “Pro” version is still in beta and free.

Put them in a git repo

I went with official Obsidian sync which also helps the dev to keep developing Obsidian.

One good thing this sync everything including keepass vault. So I kind of use it as file sync to keep keepass sync across Mac & Linux.