@fredi
As for the error "Error rendering in MediaFoundationMedia Foundation Error.” I've seen that. It occurred when I took a different video file that was encoded in mono. So I think it might have to do either with the sound or with the format of the video itself. With the GoPro files as source it works providing that you have an expected widescreen format (1920x1080 + 24, 48, or 50 FPS). It you want to use 4K you have to green screen it.
So I'm going to continue with Dashware. Even if it's a bit tricky, it's free.