stupid me. according to
https://calibresync.bitbucket.io/faqs/#q19 it's STILL one-way sync only. I was confused by changelog
original question.
Looks like CalibreSync now supports syncing back read status/date.
But how it's supposed to work if I use Calibre Sync on 2 devices and Syncthing (which does actual library sync) didn't have time to update metadata.db on other device?
What if Calibre (and Syncthing) itself is running on regular computer while I update read status/date on one of devices?
Could metatada.db ever become corrupted in this case? Could updates to last-read/read status become lost?