Thanks!
The problem seems to be solved, for now, although I'm getting failures to sync once or twice a week now. I have finally upgraded everything: newest Photos (I was in iPhoto until 48 hours ago), El Capitan (I was still on Mavericks until a week ago), newest iOS (I keep that one up to date). Maybe, just maybe, the past 72 hours of getting everything back to "just working" will lead to some kind of stability.
What I did:
- Turned iPhone off and on.
- Rebuilt Photos library: press Option + Command while opening the program. (The darn thing's only 24 hours old and it needed rebuilding?!)
- Turned iTunes off and on.
- Quit Photos.
And then it synced. Who knows which of the above, if any, made the difference in avoiding the "Photos Library is not yet available" alert. At least I didn't need to empty the iPod Photo Cache for the umpteenth time.
Of course, iTunes is telling me there are only 150 photos on the iPhone, and the iPhone tells me there 1,792, and the album that's supposed to sync has around 1,950, but for now I think I'll let that slide.
(Darn updates. My desktop photos all just reset to the El Capitan picture. Again. But that's for another thread.)