I used this same technique (fixed slow frame rate capture) for creating ‘turntable’ videos of models in-engine a while ago. This write-up also goes over the video editing part of the process: github repo + instructions
This won’t really work for capturing realtime gameplay, but you could set up an effect to fire off in front of your camera and give it a go!
Also, here’s the original script I reference which uses the Time.captureFramerate functionality.
Note, this was a couple years ago - I haven’t tested this in recent versions of Unity 