I made my own Obsidian Sync server with my NAS and a free plugin, and it works perfectly
It's arguably better than Obsidian Sync, too.

Obsidian is a fantastic note-taking tool that really clicked with me in ways that no other has before, and I can't even quite put my finger on why. It has so many customization options, is simple and easy to use, and it gives users a staggering amount of control. It gives users so much control, in fact, that you can easily host your own alternatives to Obsidian Sync, one of the few paid options that this tool offers. Thanks to my home server and a free Obsidian plugin, I now have a live, self-hosted alternative to Obsidian Sync that works perfectly.