I don't like 12.5, and am holding off upgrading my iTunes past 12.3 at the moment on Yosemite. If I update my iPhone 6 to iOS 10 via OTA, will I still be able to sync and manage my phone's music/video content with iTunes 12.3 via cable?
I doubt it, but that's just a guess. Apple has been getting stricter on enforcing supported iOS/iTunes combos.
In the past, older versions iTunes woulds still install and sync with newer iOS updates not officially supported by that version.
iOS 10 required updating iTunes to 12.5 to even perform the iOS update, so it wouldn't surprise me if it refused to connect to 12.3 under any circumstance.