Installing smoke detectors is mandatory

Thank you for the feedback

No matter what you choose now, I would at least make sure that the parts are tested.
Mine are from ABUS RM20 (no wireless) for €10 on sale with a test rating (Stiftung Warentest) of 2.1.

For me, it was important that I don’t have to drill 11-22 holes and that they are removable if needed. So they are now self-adhesive and attached by magnet.

More expensive detectors detect differently and more reliably. Pay attention to differences there. Especially if you want to network, it would be wise to choose a solution that also has investment protection after 10 years. Therefore per cable. And don't forget the appearance either. There are also really ugly ones.

I had bought Abus online at a reduced price. You can screw it on or stick it on and press the button to connect them yourself.