I wanted to make something like this (minus the gui) for a long time... thank you very much OP, I am bookmarking this.
So if you remove the GUI you'll only have the CLI, so what you want is the ffmpeg CLI?
I'm not the parent, but not necessarily. You can have this same exact "GUI" implemented as a TUI, using something like Bubble Tea[0], Textual[1], or by hand. You'd get a similar experience without having to go back and forth between the terminal and a browser.