I am pretty sure that anything you feed into the ecosystem as a personal document, using "send to Kindle" will sync . but I am not sure if K for PC can see such documents.
so take your sideloaded book, use
send to kindle to send a.mobi version to a kindle address.... that's accessible wherever you are logged into same amazon account, and I think any applied highlights, annotations that you apply to that version will travel with it. test it
doesn't solve the access issue for those you have already made though
https://www.reddit.com/r/kindle/comm...s_and_reading/