Start incorporating reading a lot earlier. I passed N4 before I started reading manga, and I regret not doing that earlier. Once I began reading regularly, I saw all of my skills improve much faster, and I had lots of new conversation topics with my Japanese friends about manga we both liked.
On the same note, try not to focus too much on the “level” of the manga and just choose what you are interested in and enjoy. I started with shoujo manga because it was easy to understand, but quickly lost interest in the storylines. But then I started 光が死んだ夏 - which some sources say are N2/N1 level (arguably) - and I loved it. Even though there was challenging vocabulary I blew through it because I was interested in the story, and it didn’t feel like studying. I say this with a grain of salt, as I’ve also chosen manga that were way too difficult and I didn’t enjoy reading it at all, but if you can find the sweet spot of interesting story and a challenge, I think that’s best. 

