The settings are now on screen with nothing hard coded. The program can be used.
Not tested on Windows & Linux, it probably won't work because we still use ShellExecute to open ffmpeg command instead of waiting for it's end.
The program is working on macOS but I can't share the binary for license reasons (the current graphical font can't be shared). I'll find a solution in the future.
The source code are available if you want to "play with it" or adapt it to your needs.