I don’t really intend to try to tell you what to do/not do, but I had pretty similar thoughts and goals when I started studying japanese, so I thought to share how I’ve been doing these past years studying for reference.
I had learned hiragana/katakana and some basic basic grammar before, but I started seriously studying Japanese in 2018 in a local summer university (don’t have to be uni student to enroll, anyone can take these courses for a fee). I studied there the whole summer, and continued studying independently afterwards. Studying independently for me meant studying new kanji in the same format which we were taught during the summer: writing a new kanji on paper, it’s meaning, ways of reading, then repeating the kanji 20~ times and finally writing a few example words/sentences where it was used (didn’t know of wanikani back then). I major computer science in uni so I spent around 2h per day on this independent japanese studying on average. I took the jlpt n4 exam the same year’s december, and passed it with good scores.
After the year turned to 2019, I found myself knowing a lot about kanji, but other areas of my japanese lacked. So I picked up some shonen manga (this was really tough and exhausting at first) which had reading instructions, furigana, for all the kanji used so it was easy and fast to google translate the meanings while reading. I started adding reading to my japanese studying routine, and at some point I remember only reading without any writing on some days.
Then come summer 2019, I find wanikani and start going forward with that on the side of studying computer science, working part time, and studying japanese by mostly reading. Then in fall 2019 I finally manage to get on a japanese language university course after basically begging the professor and going through her tests / interview (she is a native japanese person). The main reason I wasn’t able to get on a course before was because I wasn’t a japanese language major student, and there were no spots / system for outsiders to enroll for those courses (still think I was super lucky to get in). It was a yearly course for the 3rd year japanese language major students, so at first it was pretty rough.
I then continued all the before mentioned routines, but now I was also participating on the japanese course, and with that came more japanese homework. The course had 2x75min lessons per week + a lot of homework, so in total during this time I spent maybe 4h per day studying japanese on average. I then took the jlpt n2 test in december 2019, and passed it (barely).
I then continued the same routine, participating on the course, doing course homework, doing wanikani, reading manga for at least 1 chapter every day. Then the course ended in spring 2020, and I continued going forward with the textbooks we used on the course along with a friend I made on the course (we did this via skype, corona and all). We kept doing that until 2020 fall when he went on to the grad school 1st year japanese course, and I went to student exchange to Osaka university language and culture program.
When starting the exchange studies they had placement tests where we had to do tests for kanji, reading, listening and grammar knowledge, on top of writing different types of essays (more casual letters, and academical reports). I got graded as CEFR B2 (but the tests didn’t include measuring speaking ability which was naturally my least practiced area).
Now I’m studying here in Osaka, and all my classes are completely in japanese. I haven’t really had any trouble understanding the teachers, and I think I’m learning tons right now. There is some pretty challenging academic writing stuff but I just think of that as another way to improve myself, even if I’ll actually never need to be able to produce academic text in japanese. Also the speaking part has improved way faster than I expected, I already feel comfortable speaking about pretty much anything without having to guess what I’m going to be asked beforehand etc.
So as a consensus and I guess answer to your original question: for me it was possible to go beyond N2 level in 2 years by not giving up my routine even when I felt like quitting completely. Everyone is an individual, so this will definitely not work for everyone, and maybe for someone the process could be even easier than what it was for me. But if you’re willing to spend even more time learning than what I did and stick to that for 2 years, I see no reason why you couldn’t get to a level where you can go to student exchange, where the classes are taught in japanese.
If there’s anything more you’d like to ask about my journey, I’m more than willing to talk more but I’m in no way claiming that how I did things is the way someone else should learn japanese, this is just what I ended up doing in the end.
(sorry for the long post)