[stumped]

It is much simpler than you think
Once you have, once per device, enabled sync to cloud in moon, then just use moon as normal.
Assuming you are online then each time you resume a book it checks and Says something like newer position available, use it? Y/N
And bookmarks etc are updated automatically....


....ok so my instructions were not perfect. I just opened moon and checked. Do this.

With a book open to to misc options. Scroll down. You should see tick boxes for
Sync reading position via Dropbox
Sync reading position via Google drive

Tick the one you want
Do that on each device
It applies to all books, not just to the one you have open
.
I know that bookmarks get synced, I don't use notes but I assume they get synced also

( The backup and restore options on the main preferences, with no book open, are for the apps configuration.
I use that to copy my preferences easily to a new device.)

Experiment a little, ask again of stuck.
Unfortunately there is no user manual.