I used Duolingo to reach a place where I could read books in Spanish, and then I switched to books. It can’t be that bad.
The best tools are the ones you use. If Duolingo is the tool that compels you to advance your practice, then it’s de facto better than the ones that don’t compel you that you never use.
FWIW I like duolingo, but I wish they were clearer about their strengths and weaknesses. I've maxed out Arabic, and am halfway through French. I got about 1/3 through Japanese before giving up because their approach just doesn't work for it. It clearly works much better for latin languages, and even in Arabic it's great for vocab but lousy for grammar and idioms. As I said in my top post, I think phone typing is an underrated skill, particularly for a foreigner in a new country.
Nothing short of time and immersion are going to solidly teach just about anybody a language, but I have some friends who've tried it and then got discouraged on language learning all together because of the frustrating parts of the app, and the fake sense of progress that's inherent in some of their marketing =(. On the other hand I'm on a 600+ day streak and have gained a ton from the app pressuring me to come back every single day.
If you think about it, how well did you speak your native language at 4? How well at 7? Did you attend any classes to teach yourself? No, I assume not. So how come you are fluent?
Turns out, it is all about ingesting as much as possible. Even if you barely understand at the beginning. More of it - learning grammar is just a distraction. You can skim the book to have an overall grasp of it, but that's it. You weren't learning your native language grammar by the age of 7, did you?
The key is to ingest comprehensible input. If you can't understand the sentence, have a visual cues. Start simple by learning most frequent 1000 words (should take you 2-3 months max with Anki) and then you can easily switch to sentences. Just keep your senteces at the level that you understand almost everything but a word or two. One word ideally.
Read or watch only context you like, you are interested in and you have fun with. That's it. Do it long enough, every day, and you are set.
As an example, I've been learning Japanese in class for 2.5y and could not read nor speak as well as I'd like to. Not to mention I couldn't read kanji at all. Now after 9 months of trying language acquisition I can read children books (Winnie the Pooh) and comic books. If that's not a win, I don't know what is.