Step by step the most important configurations.
Settings -> Camera -> Structures -> set to "Most compatible". Otherwise you will not always have JPG format.
Settings -> ScreenTime -> Limitations -> "iTunes Store and App Store" -> Disable the "In-App buys".
Settings -> General -> Background running -> Background apps disabled
Settings -> Privacy -> Location services -> Disable as much as possible
Settings -> Mobile Network -> Wifi Assistance -> Off
Settings -> Battery -> Batterycondition -> Peakperformance -> Switch Off (if it is enabled...otherwise you do not see this option) Energymanagementfunction