Can I record with 50FPS in the camera app to avoid flicker? The iPhone camera allows me to choose between 30 and 60 FPS in the settings. However I live in Europe with 50Hz mains frequency. When I am filming old CRTs, I get a bunch of flicker either way. It would be optimal to record at 50 FPS. Is there a way to enforce this? Maybe a 3rd party app? I haven't found  one yet.
Without 3rd party apps: No, iOS doesn't allow you to choose the fps.

Yet, there are 3rd party apps that can give you much more control over photo and video settings. ProCam for example supports your requested feature.