People that use GUIs/tools for things like ffmpeg, rclone etc really want the developer to autodetect if they have it already, and use that instead of installing a separate version/binary.

How do I know? I built one (https://github.com/rclone-ui/rclone-ui)

I disagree, I don't want another ffmpeg binary, I already have one. Winget works well, especially since this is already a terminal program.

afaik winget can automatically manage package dependencies.