It is my pleasure to inform you that the research supports your conclusions on all counts :)

I fully agree with your insight on how Duolingo sets you up for failure, and it has another trap, too—one common to all methods that are based on "diligently do these drills every day"* : You think that you should be getting somewhere because it's so boring and it sucks so much. You did the work, right? You're suffering, therefore you must be levelling up. Then after 4 years of doing French grammar drills on school or French vocabulary drills in Duolingo, you still can't even ask for directions or read Le Petit Prince, and you figure it's because you're such a lazy loser with no discipline who should have drilled more, instead of spending all day browsing Instagram or playing Animal Crossing.

When actually what you should have done was to browse Instagram in French or play Animal Crossing in French. Perversely, real language learning—we call it "acquisition" rather than "learning", to emphasise how it's an instinctive, subconscious process—happens optimally when you're in a state of flow where you don't even notice you're using the second language anymore, i.e. when you're not suffering.

* There's a very limited number of things that you do actually have to consciously drill; mostly writing systems, maybe also the phonemes at the beginning (this part is debated). Luckily, almost all writing systems in current use are very simple and you'll get them nailed down in no time, as long as you already know the basics of the spoken language (remember, writing isn't made for foreigners, it's made for native speakers to represent the words they already know). The exception is if you're learning Chinese or Japanese, in which case there's no way out of drilling characters, forever. my degree in Japanese is from over ten years ago and I can read Japanese pretty fine these days and I'm still drilling characters. It is still the case that it's much easier to learn the characters the way the Japanese and Chinese peoples do it, i.e. after you know the spoken language (at least to a basic degree, say A2 or so).