So I've recently started implementing ffmpeg in an application that I do intend to distribute commercially. And I've had an hard time getting my head around the whole licensing process.
The most commonly answered question I've seen seems to be about x264 which requires a paid licence from x264.org in order to use it commercially (right?). I started to look into mpeg4 instead but that too seemed to be locked behind some patent licensing fee (https://www.mpegla.com/programs/mpeg-4-visual/license-agreement/).
I guess my question is the following, what video encoders compatible with FFMPEG are fully free to use?
Yes I barely know what I'm talking about, but I feel like I'm missing the elephant in the room here...
The most commonly answered question I've seen seems to be about x264 which requires a paid licence from x264.org in order to use it commercially (right?).
No. x264 is free to use under the terms of GPL. This means your whole code needs to be also distributed under a copyleft GPL license. It doesn't matter if it is for commercial usage or not.
However, there is a non-GPL paid license available for those who can't comply with the GPL. What matters if you can comply with the GPL or not. If you can't, then you must buy the non-GPL license.
See Choose a License for a layman's list of what you need to do to comply with a specific license.
what video encoders compatible with FFMPEG are fully free to use?
From the viewpoint of FFmpeg all of them are free to use. Again, it doesn't matter if it is commercial or not. All you have to do is comply with whatever license your version of FFmpeg is using. It is licensed LGPL 2.1, or as GPL 2 if you use the --enable-gpl configure option.
There is no paid, non-GPL license available for FFmpeg. See FFmpeg License and Legal Considerations.