Some potentially useful feed back.
It would be nice if the student could have some control of the sentences they studied. Some sentences are harder than others, I think that changes depending on what other studies people have done before they come to duo. Statically the sentences which you struggle with could be filtered off into a difficult sentences pool for a particular student. Also a number of the expressions here, are completely unnatural in there communication. They might serve in they role, to work a grammar structure but at the cost of being highly un-usable in normal communication. I would prefer to work with sentences I could imagine myself using than something I can never imagine using. Hopefully I would then spend less time with the stuff I already know and more time with the stuff want to know or am struggling with.
With regards to spending too much time on stuff you already know: it helped me a lot to test out quickly out of the lower chapters. I don't know if you currently do that, but it allows you to spend more time on things your find harder.
In some languages there are stories, which give you a lot of natural-sounding everyday phrases to use. I don't think Japanese has them yet unfortunately.