I finally noticed that CC became completely unusable after the last update. So I spent all evening last night and a good chunk of today figuring out how to get what I want with Calibre Sync.
I thought I'd share my journey in case it can help anyone else. As always, YMMV. And I'm very long-winded today, it seems.
The whole point, as far as I'm concerned, is to have all the epub files for my library on my phone and to be able to update and search reasonably well. I have 6351 books in my library as of today, just under 2G for the metadata and epubs.
First, the failures:
Smart Device Mode
To try and keep my existing work flow, I spend several hours trying to use Calibre Sync's Smart Device mode.
For whatever reason, it is
extremely limited compared to the other library modes--title and author are the only available metadata with search, but no sort capability. And plugboards are ignored, so it doesn't work to put other info in the title like I do for Kindle.
Furthermore, it broke if I tried to send more than a few dozen books across. Then it got in a state where there were entries for a bunch of books, but no epub. And the only way I found to delete those entries was send them again--successfully--then delete them from Calibre.
When I gave up, I couldn't even find a way to delete the Smart Device library other than to uninstall the app.
Calibre Content Server Mode
Worked reasonably well for metadata and searching, able to sync the metadata in a decent amount of time.
But trying to download epubs in large numbers just didn't work. If the screen locked while it's downloading, it stopped.
And there's a little toast/dialog popup for each and every download. Restarting the app was the only way I found to clear them en masse.
There is a feature to list books without downloads, but it took longer than re-syncing the library to the server.
Device storage / Internal or SD card mode with USB transfer
Given Wiggo's advice, I figured this at least should work.
But for me with Win10 and Android phone(Galaxy S10e), I could
not get a large library to copy over USB successfully. From what I read, it's a common thing with android?
And because it's a special device in windows, I couldn't point Calibre to a dir on the phone as a library, or use rsync. So reasonable update was out, too.
I considered ejecting the SD card I keep in the phone to plug in the computer, but rejected it as impractical for updates; I use a phone case.
Dropbox Mode
Same as Content Server, basically. Metadata only initially; terrible mass epub download. Plus having to upload/sync library to Dropbox.
I assume all other cloud service modes would work basically the same. Dropbox just happens to be the only one I already use.
The Winner, for me at least:
Device storage / Internal or SD card mode with syncthing transfer
This is what I eventually ended up with.
syncthing is an open source do-it-yourself file sync utility. Binaries for desktops are available and in android app store. (I am not associated with them, and I don't know anything more about it.)
I configured syncthing on win10 to make my calibre library available 'send only' to my phone where I configured syncthing to save it to an internal storage dir in 'receive only' mode. I also configured it to skip sending .caltrash and *.azw3 files.
It took several hours to sync initially--which seemed a bit slow for <2G. But it worked regardless of phone locking/unlocking, making a call, starting/stopping syncthing on either end, and changes to library while it was working.
To update, I need to start syncthing on both win10 and android, but it's all there and (so far) works reliably. I don't foresee leaving it running all the time.
It's possible I could have use Dropbox to save the library to a dir for Calibre sync, I didn't try. If I was going to jump through those hoops, I'd rather have direct sync under my control and not via cloud.
Update: Syncing for updates takes longer than I would ideally like, and the majority of it is syncthing on Android scanning the existing library dir on phone. But 3-5 minutes isn't unreasonable, IMO.