I went for the paid syncing because I want it to "just work" while still having the futureproof way of storing the data locally in an accessible way.
So far it has worked absolutely flawless. If I change a file it's changing on my connected device in seconds. Not exactly like working on a shared google doc but close enough that I would even use it as a hack to quickly share links between my mobile and my desktop
Obsidian sells a first party syncing solution, which I hear works well:
https://obsidian.md/sync
I do git syncing on Android via termux (It works most of the time, except when git decides to shit itself every now and then on my tablet):
https://forum.obsidian.md/t/guide-using-git-to-sync-your-obs...
I can't vouch for it because I don't have any iOS devices new enough to support it, but supposedly you can use Working Copy to sync via git on iOS:
https://forum.obsidian.md/t/mobile-setting-up-ios-git-based-...