[gregopet]

Sure, Obsidian is not open source, so if that's where you draw the line, fine.

But otherwise, Obsidian can be synced quite nicely whichever way you want (I use SyncThing personally), and the Obsidian developers actively encourage you to do so, even if that means they don't get your subscription money.

As for extending its functionality, plugins are really powerful, approachable and in general a joy to write (I wrote one for my private use, using the JS framework I know and not the one suggested by Obsidian, and it still works wonderful, I use it almost every day).

I really love Obsidian, matching its feature set will be no easy task for an open source project.