There’s was a scanner app that I loved, for Android. Turned into a subscription, even though most people use it less than once a month and even though the app was basically complete and never got updates.
ffmpeg is written by Fabrice Bellard, who’s one of the most underrated programmers in the world (he also wrote QEMU). It’s probably the best tool out there, still actively maintained, and most commercial apps are probably using it under the hood for any kind of conversion.