You're right about x11grab. I assumed I tested this when I added --enable-x11grab, but now that I think about it I probably forgot and just thought I did. The libx11-dev package seemed to do the trick so far, but I need to actually test it this time. Thanks for the suggestion!
Fluendo pack from the Canonical Store? I really know nothing about this software and I don't see any mention of FFmpeg in the item description.