In June of 2017, I began a study from the iPhone Photography School. It has significantly improved my iPhone photos by:
Showing hints on how to use the iPhone better
Reminding me of photography basics
Indroducting apps for camera control and post editing.
The rules of these photos is that they must be taken and edited (if they are) on an iOS device, such as an iPhone or iPad. For the most part, they were taken on an iPhone 6S plus and edited on an iPad Air 2.
Some of my work is presented in Bruce's iPhone Photos.