How can I stop the Photos app showing the "What's New Photos" screen every time I launch it?
This problem eventually disappeared in some minor version upgrade of macOS Mojave I believe (possibly even the upgrade to Mojave itself). I then had the problem that occasionally Photos.app would "forget" that I had set my system photo library to one on an external drive, which was less bad, but still not ideal. Since upgrading to macOS, that problem has however been fixed too, I'm glad to say.