Plan was to be able to have a Grafana panel, where I can see the 8 different temperature sensors + 1 Pump speed (in %).

As I have a Resol DeltaSol BX Plus, there is a bus called VBus putting out this data without the need to poll.

So, with a tiny circuit it is possible to listen to the VBus.
Circuitry is not my work and can be found online, but I modified it anyway...
(No R3, the voltage divider R1 and R2 brings the voltage down to less then 2,5V to be safe
as I measured Voltages close to 9V directly on the vBus).
What it does in the end is to shift voltage down and uses the transistors as switches to pull RX to ground or leave it on 3V3.

Total cost is somewhat around 1-2€.

some quick soldering and it looks like that:

Instead of the ESP32 I attached it to a Pi3 (actually overkill)...

I have not written the code myself, only stated what I need and a young engineer from work hacked it together for me and gave his blessing to put it up here under a permissive licence.

What it does is it uses the library from Daniel Wippermann to dissect the data stream and
a) displays this as raw content in a webserver
b) pushes the data to InfluxDB

I included a dockerfile so it is (more) easy to deploy.
I run it like:

docker run -d --restart unless-stopped
-p 11110:11110
-v vbus2influx:/etc
--device /dev/ttyAMA0:/dev/ttyAMA0
--name vbus2influx
-h vbus2influx
vbus2influx

(the vbus2influx.toml is to be placed in /etc)

Proof that the Pi3 is overkill...

A big thanks to Daniel Wippermann for some really valueable hints and providing the library
and to my coworker who wants to stay annonymous as he thinks his code is not "clean" enough ;)