There is a proliferation of sync tools and little standardization. Here some in local-first space [0]. Martin Kleppmann in his talk last year spoke [1] the need for a generic sync protocol, which was very interesting.

[0] https://localfirstweb.dev/

[1] https://www.youtube.com/watch?v=NMq0vncHJvU&t=1016s

Someone maintains a list here

https://www.localfirst.fm/landscape

LiveStore shows recreating linear as one of their examples though I haven’t tried it. It was on the front page recently https://news.ycombinator.com/item?id=44105412

Firebase (Firestore is the DB) is the best I've ever used but not open source. MongoDB has Realm which achieves similar, is open source and is OK.