[binarybanana]

>haven't had cause to use it super regularly

>I still sound a bit like an extremely out-of-date teenager.

I don't think it's surprising that the way you speak hasn't changed much when you stopped using the language. Your peers were mostly teens at the time and that's the manner of speech you picked up. Had you stayed in Germany and continued to speak with peers your age it surely would have developed along the way.