Zigbee, like zwave and enocean, also uses sleeping nodes while mains powered nodes usually act as repeater nodes to increase range of the network

If you can find a sketch that can decode the serial data from the inverter, sending those values to domoticz is very easy

I'm using them, I got the ones from soshine and the stated capacity is just a little more than the one measured by the charger.

If you want complete isolation between relays and arduino you don't need to share common ground if your relays have optocouplers.

That looks really good. Did you made an estimate of the average cost of single node?

I still don't understand what the final goal is with all those options

@franz-unix I'd go for a LDO (le33 or mcp1700) to power the board and remove all voltage regulators

Do you mean to connect multiple sensors to the same pin on the arduino?

Normal HA is ok, the problem is hass.io

Could be a lot of things, start with capacitors on the radio modules, try using other modules, etc etc