This is about the experience of learning French but it might as well also applies to Japanese.
I‘ve been studying French since childhood but only started seriously about two years ago. I progressed quite quickly to the point where I can understand enough to use immersion methods like easy podcasts, Youtube videos, stories. I have basically no discipline to study vocabulary and grammar „manually“ anymore since I‘ve started to feel like I‘ve hit an upper beginner‘s plateau. I was sure immersion is the key to overcoming that. Yet whenever I listen to podcasts etc. despite understanding +90% of it I feel like I‘m making no progress. I wonder if this is normal and I should just trust the process or if I‘m doing something wrong. Maybe it‘s also the lack of output I‘m making or maybe I should really „study manually“ again in order to gain more skills to be able to move onto more difficult immersion material. Has anyone else ever felt like this?
Plateauing is absolutely normal, and is probably a universal experience. There is also a natural progression to forgetting and relearning; the relearned material is more nuanced than naive acquisitions.
I would experiment with different approaches, maybe intentionally shaking up your routine from time to time. Many teachers, for example, have reported that they never really truly understood their subject until they needed to explain it to students.