I have the exact same problem (only difference is I have 28,000 photos). I have spent hours on the phone to Apple over three different calls, I have tried everything on this post (de-syncing, only trying to sync a few albums etc), I have reinstalled Sierra, tried syncing photos from a desktop folder instead of from Photos (Apple program) and nothing works. I even bought a new iPhone hoping that would solve the problem and it didn’t. Am super frustrated as I am a massive photo lover and reference them on my iPhone all the time.
On one call to Apple, the expert technician (my issue has been escalated to an expert) advised me that ios11 has been specially designed this way and that I need to copy photos from the years to folders on my iPhone manually. I find this VERY hard to believe. If this is the case I will move to Lightroom as it syncs seamlessly for my husband.
Apple have advised they will engage an engineer and get back to me. Will let you know if I have a successful outcome.