> I spent 18 months on it and the learning was soundly thrashed by three weeks in a classroom, and for all the pupils, not just me, primed for success by 18 months with Duolingo.

I wonder how well you would have done if it hadn't been for that 18 months. Because 'three weeks in a classroom' sounds suspiciously to how I learned Polish: I seemed to be making zero progress for years and then in a very short amount of time (a couple of weeks at most) it suddenly 'clicked' and I started to make sense of what I heard. I probably was learning at some level that wasn't accessible all the while thinking I wasn't making any progress at all.

Duolingo is 'infotainment', it gets you from zero to a little bit above zero and it may give you some useful number of words to recognize and maybe even to use actively. But it won't help you to speak a language, for that you need to progress beyond it. This is pretty much the same story with every other language, nobody learns to speak and use a language actively by reading the dictionary. But it certainly doesn't hurt to have a basic 200 or 300 word vocabulary on which to build.

Personally I use Anki for such learning rather than commercial services, it allows for far more customization and you won't be sucked into silly gamification for the sake of extending your subscription because there is none.