Allow me to clarify about "in-app purchases".
The "in-app purchases" are for small complementary features, like making the screen appear on a schedule, making it impossible to skip the screen, and adding a lock button to lock the screen. Those features aren't essential for the app to function.
> This app could just be an image set as your lock screen background.
Well, yes and no. In the app, you can interact with the prompts. There is a history of your itneraction. You can export it and then analyze it if needed.
> I've found a good way to discourage mindless phone staring is to set the display to monochrome (e.g. through colorblind emulation). The decreased visual stimulation seems to have an effect on me, at least until I want to see a photo or video in colour and go back to normal.
+1 here. I have always had this setting on closer to bedtime.