I spent nearly 3 hours on Marble Track 3 today, but over an hour of it was trying to figure out how to make Blackmagic Fusion import multiple sequences of images to put them into one movie.
I was not able to figure it out.
I have a sneaking suspicion that BM Fusion is not designed to do this. Most of its output options were images, so.. apparently it is designed ot output images instead of videos. It did have an option to output quicktime, but the default was jpg.
Since June 2017, I have spent 5.7+ days on Marble Track 3.