Just to clarify the entire Logseq app is open source including the sync mechanism, the server backend to receive the sync endpoint and store the data isn’t. I use Syncthing (FOSS and cross platform) to sync noted between my devices.

The whole software is currently considered alpha. If you want to test bleeding edge stuff, you could build the main branch locally and test that. Or you could use the app releases and report bugs on github (the devs would much prefer pull requests fixing bugs than just reports and they’re great at merging). I’m not a dev but I have contributed to the app.