Also a lot of it is essentially wrong, since they expect you to transliterate parts of the sentence that would probably not be included in context, e.g. saying "watashi wa" for any sentence starting "I".

I don't blame them for that. It seems to me (a very early beginner in learning Japanese) that a lot of languages have a formal way of expressing things, and informal shortcuts which native speakers use almost always in real life. But when you're just learning, you need to learn what "wa" does and what "watashi" means, and thus having "watashi wa" may help.

It's like in martial arts you learn kata first, though if you get to apply it later you don't do the same kata, you do something else - but to do it, you have to learn to do kata first.

While there may be some specific cases where it's done, I'm halfway through the course and watashi was shown on its own, (quite early) but I don't think it was used in even one sentence so far. (Most of them starting with I)

I've made it through the full course. It will generally let you add "watashi wa" on, but rarely mandates it. Most of the time the suggested answer won't include it either.

> stuff they randomly throw at you without any explanation

That's kind of their approach. I agree that in case of Japanese they could do better. E.g. if they explained what is explained here: https://8020japanese.com/japanese-sentence-structure/ - and they have space for such texts, but they are not using it for Japanese - it would save me some frustration and trying to figure out what these weird things are doing inside the phrases. Once you get the principle, it kinda clicks in place. So with the help of some other sources, so far it's not too bad for me with Duo and Japanese I feel. Though of course I don't think it's possible to use it alone without some auxiliary materials.