iTunes cannot sync photos to the iPhone “B’s iPhone” because your Photos Library is not yet available. Please try again later.

Editing was easier with iPhoto.

Actually, Photos has more editing tools than iPhoto. Perhaps you have not yet found all of them?

The new Photos for Mac has more adjustments than iPhoto, but you have to reveal them. Initially it will only show very few adjustment, with the advanced options hidden.

For example, to darken the highlights or brighten the shadows, click the Adjustments button, then expand the "Light" controls: in Photos you will find the Highlights and Shadows here:

• Select your photos, hit the return key to enter Edit Mode, then click the adjust button (the knob with checkmarks)
• In the Adjustments pane click the blue text "Add". Now select all adjustment you may want to use.
• Then click the "Light" adjustment and click the down arrow to the right of "Light". Use the Highlight and Shadow sliders, also try to use "Brilliance". It is a nice combination of both.

If you hover the mouse to the far right of "Light", you will notice hidden controls - Auto and a downarrow. The downarrow will disclose the advanced adjustments permanently.

Just in case you are wondering where the straighten tool is hiding, to straighten the horizon: It is combined with the crop tool. Open "Crop" to straighten the horizon. The Auto button for straighten will try to find the horizon automatically.

I'm using this all the time.

To blur edges, use the noise reduction tool.

I have close to 50,000 photos in my database and that may be the cause of the problem.

The size of the Photos Library should not be a problem. Photos is supposed to be able to open a library with 1000000 photos, just like iPhoto and Aperture, since it can migrate iPhoto Libraries and Aperture Libraries.

But after the upgrade to macOS Sierra there will be short period of a few days where Photos needs to prepare the Photos Library for the new features. It is analyzing the photos and classifies them according to categories. Afterwards you can automatically search for photos by the content of the photos, without having to tag the photos with keywords. Search for "beach"; "sunset", dog, "Birthday", "Wedding", for example. This analysis needs a lot of processing time for a large library.

For example, when I search for "Wedding" in my library, I'll see search results like these:

I never tagged the the photos as "Wedding" photos - the categories have been assigned automatically from the items shown in the photos. and occasionally Photos will get a picture wrong, but that is artificial intelligence 😁

O yes, the System Photos Library is a strange beast. It is usually always in use by the system background processes watching and updating the library while we are logged in. That is one of the reasons why it is nearly impossible to share a system photo library with someone else on the same computer. The safest way to get access to the system photo library again is usually to log off, and then to restart the computer for good measure. Since OS X Lion applications may not really quit when we quit them because of the "Automatic termination" process model (https://arstechnica.com/apple/2011/07/mac-os-x-10-7/8/#process-model). When we quit an application, the system takes that only as a hint that it is safe to quit the app, but may keep it cached and not really quit it, even if the blue dot vanishes from the Dock. Logging off will clear the cached versions and ensure that Photos will really quit and write all changes to the library and the preferences files. And restarting will terminate the system processes.

Once in a while the "Photos Library is not yet available" prompt is actually accurate - the library is (I guess?) processing new images and it doesn't feel ready to sync yet. I had this happen a few days ago when my partner finally synced photos from her iPhone to our computer, and with 2,000 new photos it couldn't sync properly for a couple of hours. Later in the evening, sync proceeded without problems. So if you've recently added a bunch of photos (or if you've recently updated the macOS), perhaps just take the message at its word and try a few hours later.

Mind you, that was one time out of the dozens I've received that error message. The other times I've solved it by some combination of closing and opening Photos and iTunes, turning off and restarting my computer and iPhone, and twirling clockwise three times with my left hand on my head and my right hand clutching a talisman.