I find that the most productive Duolingo exercises for me are those in which I have to simply translate directly from my native language to the foreign language.
Those exercises accomplish much more than foreign-to-native translation, picking words from a collection, picking the word that describes an image, etc.
I would find it extremely helpful if Duolingo had a difficulty slider, or even a binary setting, that made it possible for me to do just direct native-to-foreign translations repeatedly.
I think you're underestimating how demotivated you can become. Doing a single repetitive exercise may become boring, reduce the chances of you learning effectively, and ultimately make duolingo worse. Most individuals here are not educators, so we(including myself) can't appreciate the need for various kinds of exercises.