Connected smoke detectors, but which ones?

Then just take those? If it should be as cheap as possible, set the price sorting on Amazon to ascending and buy the first one. Whether you can then integrate it into your ioBroker or whatever else, you have to find out yourself. Usually, unless there is a direct KNX connection, you take ones with radio networking and switching output. Then you let them all trigger together or by floor and connect the last one in the group to the building control via binary input. As far as I remember, your systems in the house are chaotic anyway.

it's chaos anyway with the systems you use in the house

That's probably true, although currently only EnOcean and Doorbird are in operation.

I get the Homematic connected via the Raspberry Pi. I don't necessarily need the integration into the system; what's important is that they are networked. So I thought there might be an alternative for 20-30€ that works flawlessly and maybe simply doesn't have integration into the system.

I have the Ei650iC from Ei Electronics, they can be connected by cable. They are also available with radio modules, but these are more expensive. So far, I have had no false alarms.