@Roycefer
Honestly, for B4J I haven't had any needs yet so I don't know much what is available. For Android instead, I have wrapped an ffmpegencoder which works fine (using command line api).
However, that said, I have already used ffmpeg with B4J/jShell for streaming and encoding but not for image/audio manipulation. It works fine so it ought to be fine for other purposes as well. You could of course write a wrapper around the ffmpeg command line tool and then expose to the users various predefined custom methods of different nature. This would also make the library very flexible.
I had a look around and indeed the "humble-video" project seems to be very rich and promising although, as you say, it looks rather complicated.
I saw some other (javafx) projects, based on ffmpeg, which look interesting as well such as :
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https://github.com/TheTrio/SlickConvert
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https://github.com/marcelluiz96/Aljava-Portable
Maybe you have already tried them?
Sorry I can't be much of help in this moment but if I come across a project which looks promising, I will let you know.