for debug, I had to add this:
.\adb shell pm grant org.mozilla.fenix android.permission.READ_EXTERNAL_STORAGE

then, the addon works well, but what does this mean for end users installing the addon?

This is not directly related to addons, this command simply allows Firefox app to read external storage.

And since during the development the extension is stored in external storage, you need this permission to allow Firefox to load it.