Yes, DIN is good and know about Phoenix Connect.
Now considering
PCF8574-I2C-Bidirectional-I-O-Expander-Relay-Control-SmartHome-Ardiuno-Raspberry https://www.ebay.com/itm/282692860765 & 8-Eight-Channel-5V-Relay-Module-Board-with-Optocoupler-Power-Supply-PIC-ARM https://www.ebay.com/itm/322834065204
It looks like a straight IDC cable would connect the two boards. But that's 2 boards to mount per bank, and not keen on depending on eBay as a supply channel.