Semi-detached new build, which technology to install?

Building a new system based on Osram wireless sockets, Xiaomi stuff, and Hue bulbs is, in my opinion, the completely wrong solution. You can do that if you live in a rental apartment and absolutely cannot change the wiring.

Amen. By now, I've seen so many botched installations where allegedly everything runs perfectly that I can't even count them anymore. Of course, the owner is proud of his work, and clearly, to each his own. But often, it's a hodgepodge of IoT that "somehow works."

OpenHab didn't sound that complicated after all. And I'm not technically incompetent, but since I'm neither an electrician nor a computer scientist, I'm a bit uncertain

At first, it all doesn't sound that complicated.

We have Home Assistant together with Z-Z-Wave devices, Shelly, Xiaomi, and others. With Shelly alone, you can already do a lot, it's really great! Especially with Home Assistant, you can combine a wide variety of devices. We have equipped the entire house for about 500 EUR (important lights, motion detectors, door/window sensors, blinds, smoke detectors, humidity, temperature, partly ventilation).