But Obsidian doesn't even require a few dollars a month for sync. You can use Github or whatever sync service you want. Your data is just a directory of markdown files. That said, I've paid for the sync service for years, but only because it works really well on Linux. I've always been impressed with Obsidian's first-class Linux offerings.

it's nice to have open source implementations so you can extend / borrow / merge / morph features and ideas into something else