I will definitely follow up again.
I actually don't care at all about the cleaning options. If I can't sleep, I don't feel like cleaning anyway. The question of what is still possible afterward is something we are also concerned about. Unfortunately, the window manufacturer completely brushed us off. I've already looked for end caps or something similar myself, but I haven't really found anything. Making something up is too uncertain for me, I don't want the shutter to spread apart eventually and something to break. We now have blackout curtains, but they have to lie nicely and precisely, otherwise too much light still gets in. It’s never really nice with any "DIY solutions," especially in a new building.