Hi Florian,
yes I replaced the orginal Board with the RSR-200 like described in your photo.
Then I used the 2 big pads of the button (on the backside of the circuit board where the red arrow in your photos points to. To solder cables to the pads, I removed the dark coating with a cutter knive.
For the wiring look at the section “Analogue intercoms” in this document:
You need yellow (ring detection), blue (door open signal) and purple (GND).
Between yellow and purple there’s an optocoupler in the Opener that detects incoming ring singals.
It seem to work with a wide voltage range, in a test the Opener detected 3V signals as well as 24V singals as a ring event.
Between blue and purle is a solid state relais that connects blue to GND when the Opener wants to open the door.
Be aware, that this means both functions use a shared GND (purple wire).
Luckily the button inputs on the intercom use pull-up resistors in combination with buttons that connect the input to GND when pressed.
Because of this, one of the button pads is the GND signal.
This allowed me to avoid additional power supplies and connecting other voltages to my Intercom system:
I use the BUS+ for the ring detection and switch it over the relais contact.
So the wiring looks like this:
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One wire from BUS+ to one of the relay contact clamps of the RSR-200
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Yellow wire from the Opener to the other relay contact clamp of the RSR-200
This results in an open input when noone rings, and 24V when a ring signal is active.
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Purple wire from Opener to BUS- (GND)
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Blue wire from opener to the button pad that is not GND.
That way, the Opener connects the button input of the intercom to GND,
just like the original button.
Then, in the app select Generic/Analogue for your intercom type.
In case you want to use your exisiting intercom station instead of a second one like I did:
Maybe you can see where the signal of the button input for “open door” is leading to on the circuit board and solder it on the other side of the circuit path.
Because if you solder it on the button pad, you might get problems with the original button.
Maybe this can also be solved by cutting some of the plastic of the top-part of the button where your wire is soldered…
I hope that helps, as Nuki doesn’t provide information about the internal connections of the opener on the website or in the app.
But the support team was very helpful when I asked by mail. 
If you have further questions, just ask.