I'm trying to find ways to reach the C2 level in English.

"the C2 level" is not incorrect, it's just more natural to say just "C2 level"

It's really problematic for me because I'd be so upset if my English ends up getting worse aswhile I'm focusing on another language (German).

To be clear: "as I'm focusing on another language" isn't wrong, and it would be normal to say it like this. However, it's ambiguous whether you mean "as" in the sense "while" or "because". To add extra clarity, you could replace "as" with either "because" or "while". "ends up getting worse" is a very natural and idiomatic phrase.

My strategy is going to be on focusing on more qualitative English material to ensure that I learn more complex vocabulary.

I removed the "on" from "on focusing". However, I wanted to also point out that "My strategy is going to be" rather than "My strategy will be" is very idiomatic and sounds natural. Kudos! I am not sure exactly what you mean by "qualitative" so I didn't correct, but it's possible that "high-quality" would be a better choice (if that's what you mean). Qualitative in English is typically used as the antonym to "quantitative", for instance "qualitative data versus quantitative data".