I have found a way, it is possible to subscribe to Telegram Premium at a cheaper price and in Germany (where it is actually not yet officially available)!

1. install Telegram Desktop or Telegram Android (but the version from the Telegram page).
2. write to the @PremiumBot.
3. pay for Telegram Premium with a credit card.

I think the 4€ per month is a reasonable price for me for now. I’ve been using Telegram for years and I’m thrilled how quickly new features are implemented. I will gladly pay to keep Telegram going for a long time.