If you are up for experimenting with AI, I used recently the https://runwayml.com/ to remove some backgrounds. But I guess for your purpose a bit too experimental… As the other stated above, it needs several tweaks to get a proper key an Natron.
maybe this might help: Natron/tutorial-AlternativeMatteExtraction.rst at master · MrKepzie/Natron · GitHub
btw.
curious about the resulting vid you are doing 
Edit:
maybe make a google search for: “OFX Plugin Keyer” you might find and test (and report here) things like:
or