Never connect the grounds of the DC voltages together.
Everything that does not have the same ground as the power supply of the RPi must be connected via optocouplers.

If all grounds are already connected, then you should get along without potential separation. But this is usually not the case. I would do the analog measurement on the high side. A very simple variant would be to measure the voltage with an attiny13 and transfer the measured value serially via an optocoupler to the Rpi. And that for each circuit separately. Then there are no more ground connections.

Save the script directly from Windows and test if it can be opened in Domoticz.

WizNET W5500 Ethernet module

For your information.
A W5500 module can also be used. The wiring is the same as for the W5100 module (with A0, A1, A2). The Arduino code must be changed in two places:

1. use as Ethernet module type
   #define MY_GATEWAY_ENC28J60

2. #include <Ethernet2.h> instead of #include <Ethernet.h>