Even though I do speak German, I would appreciate if we kept this conversation in English, so anyone can read along and contribute - or, at least I will answer in English for that reason 
To your question:
I completely forgot to take pictures while building it, but here’s a quick and ugly schematic of how it’s all wired up:
Usually boilers have a hollow rod in them, where a temperature fuse is pushed in - you can use the same opening to put the temperature sensor in too.
And a close up of the ESP32 and MAX485
red is 5V, pink is 3,3v. black is GND and green, blue and yellow are data
I completely took appart the Dimmer, removed the outlet and added a housing in its place to put all electronics in, so there’s not much to see from the outside any more.
Here a picture of the dimmer’s internals. It’s really well built and easy to work with.
There’s one cable (with 5 cables in it) that delivers mains power to the dimmer (and ESP) and feed the dimmed power back into the boiler. (in the final setup I don’t have a temperature sensor integrated there, but get the temperature readings from the modified boiler)
But here’s what it looks like for me in the end:
and yes - I’m also a bit unconfortable with how close the plug is to the water inlet of the boiler (even though it’s completely tight of course) … I will place the plug way up higher some time, but to do that I’ll have to pull a new power cable from the fuse box to there … which I was too lazy to do now
