Plenty of room on the tablet. The 4000 ePubs takes up less than 0.5 GB. I used small ePubs for this test. Mostly around 100kB. And I have 18 GB free atm on the internal "sdcard".
Guess what? After around an hour it seems that it had synced after all. No output in debug while it happened. Or what was going on while it seemed that it had hung. Perhaps I should have specified some "debug-level"?
I have "Automanage metadata" on, that might have been it?
So it seems that reconnect works, but is rather slow...
Once it had reconnected and synced it was fast to add a book.