Just a couple things (okay, sorry, 80 million things). No one ever becomes a "native-level" second language user (or third, fourth, etc.). Native speakers of that language can always tell, so don't worry about that being a reasonable goal. Like the OP says, use Japanese for your purposes. For example, you might want to reach N2 level on the JLPT, or learn enough Japanese to get around while traveling, or communicate with your friend in Japanese, or be able to play videogames-- all good reasons. Reasons that will lead to failure are: Japanese seems pretty cool, I think I'll pick up Rosetta Stone or I want to truly master something in life, so I'll become a native Japanese speaker.
Also, the bit about learning Japanese for media might be a little misleading. Consuming media relies on listening and reading, absolutely, but the learner still needs to produce the language in some way, speaking or writing, to learn it. Just in case anyone misunderstood-- people can't absorb a second language like a first language (many still believe this!).
Now I'm going to share what "ingredients" you need to learn a language. The following is just ideal circumstances, the rest depends on what you, personally, put into it-- time, commitment, motivation, and finding/working with your strengths and weaknesses. I am sharing this with you because I've had terrible and wonderful experiences learning Japanese revolving around these core elements, and I am also getting my MA in TESOL and am up to my eyeballs in theory/practice about second language acquisition. So here's a crash course on generally agreed upon important factors in learning a second language, but more importantly, Japanese!
1. Input-- "taking in" language-- is never enough, but vast quantities and frequent exposure to input at a level YOU can understand is still a deeply important factor. Luckily the internet exists, so if you're a beginner you can try to find things with lots of pictures and diagrams, or short passages in text or short audio clips relevant to you. Intermediate learners can use news broadcasts, TV shows, newspapers, etc. And it should be as authentic as possible-- Japanese that people actually use.
So input needs to be understandable (at your "level"), relevant (you only learn things you need or want to know), and authentic as possible (movies/books/games, while scripted, still aren't written like textbooks!).
2. But more importantly, OUTPUT. People need to produce the language in some meaningful, relevant way to really learn it, even if they're just going to watch TV shows. When you write or speak, that's how you notice gaps in your knowledge or ability, and that drives you to find the language you need. Simple drills out of context (mimicking or copying language) isn't enough because that doesn't push you to use the language in a real way to the best of your ability. The OP mentions Lang-8 which can help you produce meaningful output in the form of journal entries that are read by other people. Ideally, you will find ways to express yourself in Japanese to communicate with other people.
So, with output you should ideally be pushed to use the language to get a message across that needs to be understood in a context meaningful to you. Drilling is fine, too, to help you remember the non-contextual components (like how to write kanji or just the Japanese characters!)
3. And that leads to the last main component: feedback. People need feedback when learning a language or else they have no idea if they're learning it. You can listen to Japanese every waking minute or talk to a wall sunup to sundown, but at the end of the day you need to communicate face-to-face to get those "sorry, could you repeat that," moments or get direct feedback about language you're having trouble with like during classroom instruction. Basically, you need to "negotiate" meaning or language points for it to really stick. You need to talk about what you're saying.
Ok, so to recap all that, the three main language learning "ingredients" are:
1. vast quantities of (ideally authentic) input you can understand
2. output, either controlled or "pushed" to get a message across
3. feedback, either indirectly, like puzzled expressions or appeals for clarification, or directly, like someone correcting a word you said, or talking about the language in a classroom setting.
Ok, now story time illustrating the importance of creating the right learning environment.
This is why I didn't learn anything taking courses at college in the US for two years and learned a million, billion times more studying abroad in Japan living with a homestay family for one year. In the US, the only time I ever used Japanese was in a classroom. The textbook was a nightmare: Roman letters only, with each chapter being a new grammar point (like a verb tense). Huge red flag because Japanese people use JAPANESE writing and people do not speak in ONE fucking grammar point at a time. Feedback was minimal, because we just sat there and listened to perplexing grammar points without real context.
In Japan, Japanese was everywhere so there was tons of input. My homestay family did not speak English so I was forced to speak Japanese to communicate and get along with them-- and that need to communicate triggered a frenzy of learning because I'm the type of person who needs to get along with other people. Feedback mostly came from my hilarious younger homestay brother who would NOT cut me slack like adults. I encountered at least 10 "WTF are you talking about" moments from him a day. We talked a lot! Classroom time was immensely helpful, too, because we learned Japanese in the context of functions you need in daily life and focused on controlled practice with other students-- we didn't sit there, dumbfounded while we listened to grammar being explained in English. it was still grammar focused, but used in an authentic context with tasks in the classroom made a huge difference (in fact, we used the Genki series coursebooks!)
However, this is SUPER IMPORTANT: the environment change in itself was NOT the reason I learned so much Japanese. In second language acquisition research it's been shown that there is NO correlation between language proficiency and length of residency. You can live in Japan for 20 years and not know how to order a hamburger. You can live in your parent's basement and emerge a nihongo pro.
The point is that people need to CREATE an environment that helps them learn the language. You can create the environment by living in Japan, seeking input in the real world and communicating with Japanese people face-to-face, OR you can take advantage of technology and find the input you need, the resources to help you learn, and Japanese users to communicate with online.
I mean, a class will help all of this, at least with getting feedback, but also keep in mind that most language learning happens outside the classroom. So, to get back to my HORRIBLE first experience learning Japanese, I could have gotten so much more out of the class if I had known the importance of creating the right environment and how to do it. I could have scoured the internet for input, I could have befriended Japanese exchange students on campus, I could have come to class with questions for feedback, but I just didn't know any better than to punch in and out of class.
So you can do it, and I hope I helped guide you a little bit. Like I mentioned earlier, I only shared the "ingredients" for language learning; only things about your environment. We all have to learn about and capitalize on our individual differences when it comes to learning a language. I'm sure others will offer insight in that regard.
