An under one roof photo program is ideal, and LR and C1 are clearly capable. However, some people might want to use the attributes that Photos does have, but would also like additional editing capabilities for their special photos.
If you like Pixelmator and it can use Photos as an external (referenced?) library...might work. No subscriptions, perhaps trying harder.
Hopefully somebody will try this combo and report on it.
It's dead simple, just not as efficient as other products. Just use Pixelmator to open a file in the Photos Library, edit, save. If you want it in Photos, just import it. Not any more difficult than if you got a photo via email, or copied it from a thumbdrive, or whatever.
And that's true with tons of other applications as well; just use "Open..." in the application's File menu, navigate in the dialog box to "Media," and your Photos library should be there. Works with Acorn, but not 2008 MS Word.
Once you've saved that edited photo just control click and use the Share... item to add to Photos. Or open Photos and import.
Obviously lots of more capable applications can watch folders, automatically import edited photos via a round trip, etc. But it isn't too burdensome to work this way if you wanna use Photos.
BTW, take a look at Mylio. It can automatically see that you've edited a photo in another application and import it; it doesn't require a plugin. Basically, it watches any imported folder. So if I edit an image in that folder (even if I do that without even opening Mylio itself) using say Pixelmator, edit, save a JPG, Mylio automatically adds that JPG to its photos. And it can do that even with referenced folders from Lr if you use that.
