Here's my current workaround for reader syncing between Winodws and Android.
I use FBReader on Android, with the FB Reader server (
https://books.fbreader.org/catalog) providing syncing across devices.
And since I can't find any way to sync FBReader with Calibre, I downloaded and installed the BlueStacks Android emulator to Windows, loaded FBReader into the BlueStacks Android environment, and voila! I can read via FBReader on my laptop, with syncing to my Android and iPad handhelds.
The BlueStacks emulator is slow to load, and needs tweaking with BSTweaker5 to get rid of some game suggestions, and works better if virutlaization is enbabled on the laptop. But it's the best route I've found.