Downgrade iPhone 12 camera to a single lens, with an app?
The iPhone 12 only uses one lens at a time to take a photo, so essentially what you want is already what you get.
Remember that at the bottom of the standard Camera app, you can switch between "Photo" mode and "Portrait" mode - and that with the ".5x / 1x / 2x" indicator just above, you can switch between lenses.
Perhaps it is the new lens correction feature for the Ultra Wide lens that is annoying you. You can turn that off from Settings in the Camera section where you'll find a slider for Lens Correction.