Suggestion: Option for "Strengthen Skills" to prioritize by most forgotten words.
As a learner who's completed a Course to the end, I would like the option for "Strengthen Skills" to choose the lesson that has the words I have not seen for the longest time.
Currently, "Strengthen Skills" selects the first non-gold lesson (starting from the top). At the rate I use duolingo (a couple lessons a day), this results in the lower lessons never getting worked.
My work-around is to manually select the weakest lesson when practicing, thus never using the "Strengthen Skills" button as it's clearly intended to be used.
My belief is that this button was only design to be used once all your tree is gold, and then when some random skills, from any point along the tree, will begin to fade, it would target them (from bottom up).
That is the only way it makes sense to me. When your tree is not gold it's effectively broken and useless.
So you must keep your tree gold in order to really use this feature.
You could install the user script "Skill strength viewer" and pick those skills, which have the least percentage.
If you would do the DuoLingo course in parallel on Memrise, you get some better review intervals.
If you would use Anki SRS, Flashcards Deluxe (supporting Quizlet import), you could have luck and you maybe find a shared deck which already contains some sentences as well.