Thanks @brandon92 for your reply and your link. It's very interesting, but a bit overwhelming for an electronics newbie.
First off: A few pictures about my current setup, it's kind of a prototype, but actually I expected to have a bit more success with it.
Left: 5V Power Supply
Left-Middle: Connectors for 230V load and some of the sensors
Middle Top: Raspberry Pi, 230V relais (currently running from GPIO directly)
Middle Bottom: Level Shifter (bi-directional), MCP23017 Expander, non-used relais
Right: I2C AC Dimmer module
Most right: Connectors and Resistors for DHT- / SHT-Sensors
Maybe important information: The 230V wires go to the backside of the wood. I hoped to reduce interference that way. The wires for the sensors are running through the AC-dimmer, which I now guess is far away from optimal.
There is also a Neopixel strip attached which is running via PWM. It's placed on the far left side and not visible on the pictures. Same for three hardware buttons and some water-level sensors.
In the worst case, I could remove the AC-dimmer and go with simple on/off relais, but I would love to be able to use the dimmer instead to reduce the power of the connected lights if needed.
Is that information helping? I'm currently thinking about replacing the simple relais with an I2C Relais board and try to put the two I2C devices (relais and dimmer) into a shielded case or something.