I had a similar idea (if not the same), while learning about ffmpeg a year or two ago, and quickly put together a small prototype of it. At least, for the GUI command builder part of it.

Here's the demo: https://jonbo.github.io/project/simply/

There's also a small ffmpeg example. (If you click the fetch button, on the demo.) The schema "recipes" for it are just a basic JSON format and can be seen here https://gist.github.com/jonbo/c4067cd18e5fa687e896b2358aaf9e... and https://github.com/jonbo/simply.recipes/blob/master/docs/REA...

I never did publish the (non-bundled) source code for the demo, since it was done in a hurry (spaghetti), but I might if there's interest.

Maybe this will inspire somebody else to build something better!