I probably don't know how to solve the problem, but I don't even understand the problem as I haven't used http://linear.app. I think you would get more constructive answers, if you would elaborate a bit on what "realtime sync engine" means?

https://replicache.dev/ Realtime sync would mean sync data across platform in realtime. replicache is one such example. The apps like figma, superhuman also use this kind of implementation..

On a quick browse of the front page, I suppose you could hook replicache to the fulcro db, so that whenever replicache sees changes, the db is changed, and any mutations would be similarly transferred