Ffmpeg as a huge dependency tree (more than 200 libraries). Upgrading it could break a lot of things. It would be trivial if it where in buster-backports, but it’s not (probably because it would imply backporting a lot of libraries).
I strongly advise you to wait for the publication of Yunohost 11, or to upgrade to the the last RC if you can’t wait and are ready to face some remaining bugs. It would be a lot easier and safer than upgrading the ffmpeg packages.