A few days ago - I had some strict requirements of extremely low monthly cost (I only need it for sending and receiving SMS and for signing up for certain german services that require a German number) - so I want with O2 Prepaid. There are a lot of instant sign up eSIMs that are data only, which is not what I'm looking for.

I would be surprised if you're talking about an eSIM service that give you a phone number.

Absolutely, the data-only ones don't count :D Fraenk [1] is one example, there are a few competitors all in about the same price range

[1]: https://fraenk.de/esim

Thanks for that, pleasantly surprised.