Which shape for the venetian blinds?

Do you mean CDL?
I guess the price is the downside

Sorry ....yes exactly

Would you prefer cdl over the zlamelle?

Roma presents CDL as the best, so it will probably also be the most expensive version. I only know the CDL from pictures. The Z slat already darkens well, CDL seems to me to have rather optical reasons (plus a bit of pseudo security). We ourselves have so-called T slats, which overlap similarly to Z slats and are therefore better at darkening.

I would basically avoid simple or round ones.

Have you installed them yourself yet?
Maybe you can give me a few tips via PM.

How is it with slats when it’s windy? I always thought that especially for bedrooms, Venetian blinds are unsuitable because they can rattle quite loudly at night when it’s windy.