ObsidianMD

There's no reason to pay for sync. As your vault is plain-text files you can use dozens of existing sync solutions.

There's an open-source community sync plugin which aims to provide an equivalent replacement to the paid sync: https://github.com/vrtmrz/obsidian-livesync

I'm happily syncing my vault with Drive, and also paying for the sync as a way to support the Obsidian app development.