Just spent about an hour trying to find which line of code works out of the multiple offerings on that link. Do you happen to remember which one you ended up using?

I just used OpenShot to do my videos in the end because KDEnlive isn't very quick at the old rendering. Stretched a still image manually across the length of the audio and exported. Took about 10-15 mins to render compared with nearly 2 hours in KDEnlive!