Also, there is no company anymore. It went out of business.

Nice, I at least received an automatic reply now from info@denic.de

I wrote them a mail which was linked under "general request". I just noticed it was in even in December 2022, not January 2023.

I just wrote them again asking if they received my mail and forwarded that to their info@denic.de address. Just saw they have more contact info on their website.

If they still don't reply, I'll give them a call. I won't give up that easily haha

You should contact the agency to transfer the domain or wait until the domain expires (however if they have auto-renew and don't bother with the extra money leaving the company's pockets it will not happen soon).