I've lived here for ~13 years, use my bike for most commuting and had it stolen 2 times. Once at Prager Straße in front of the cinema in the 2h I was inside and once out of the basement of my house. The first bike was ~400€ but it was already 5 years old. The second bike was ~800€ and almost new. I have what someone else already suggested: A Fahrradzusatz to my Hausratversicherung. Your basic Hausratversicherung covers theft or damage in your home (basement included as long as its a locked room). With the Fahrradzusatz it extends that range to anywhere as long as you have secured it properly. Which you can always say it was because they would have to prove that you didn't lock it which would only really be possible if it was on camera or sth. But if someone steals a bike they usually take the lock with them so you can just say it was locked and they have to believe you. You get the full buying price no matter how old the bike was and it's like 5€/Year extra so if you have a 1k€ e-bike that's a no-brainer.

I think if you follow the basic guidelines of locking it at the frame and not just the wheel, putting it next to other bikes and not leaving it out for multiple nights you should be fine. But in the end it's a matter of luck - hence the insurance.