Although I never used it, I am aware that Calibre can serve books in your local network. I imagine that this offers some position and annotation sync.

Also, a bit off-topic for this sub, but… how do you read? E-readers? Tablets? Software choices?

Unfortunately, there was never great ebook hardware. I use a tablet with Android. KOReader for ePub, constantly trying new Android PDF readers but finding nothing decent.

While not intentionally, running Syncthing between all my computers means that my PDF annotations get synced across devices. ePub ones do not; afaik KOReader uses its own metadata format that it stores as a standalone file.

Before, when I was still in university, I used Zotero also for annotation management. Feels like an overkill nowadays since I only read for leisure.