I dig the simplicity, but "mp4" for example is just a container format. Even if you don't make the codecs user-configurable, it'd be nice to know which codecs are being used e.g. "mp4 (h264 + AAC)".

Question, does it strip metadata? Because ideally that's what I'd expect from a "privacy-friendly" tool.

I let it "convert" one mp4 file into another, and it's taking a while, which makes me think it's doing a full re-encode. It might be worth adding logic to merely remux i.e. "-vcodec copy" where possible, as an optimisation.

Consider adding HEIC/HEIF image support, as well as JXL. Slightly less common, but all the more reason why someone would want to convert.