Updated June 12th, 2019.
If you don’t want the Photos app to launch every time you connect your iPhone all you have to do is uncheck one little box in the Photos app. Note that this setting is per device– you can uncheck the box for one device and leave it checked for another.
The button is in the Photos app’s title bar, at the top left. First you have to click on the iPhone’s icon in the left-hand sidebar for Photos. Then you uncheck “Open Photos.”
That should do it!