Self hosted obsidian live sync eorks very well. I use it on 3 devices each with a different OS. Super fast and responsive. I can see text appear almost as fast as Google docs when types on one device and watched from another.
I know obsidian itself is not FOSD but the files are markdown and you can easily back them up with your existing backup setup (seafile for me) and then open up the plain md files if needed for some reason