Is anyone in Germany able to recommend ISPs which aren't Vodafone or Telekom?

No. My company went with a so called better ISP because doing AI means transporting tons of image data up and down. But their peering was horrible. I couldn't update Ubuntu at all compared to fibre Vodafone at home. Took hours. Transporting the harddisc home and back was faster.

We complained a couple of times with traceroutes showing their nonsense routes, and eventually it got a bit better. But not as fast as Vodafone still