I failed to emphasize on one other thing in your system. You can not measure both the solar panel voltage and the battery voltage without isolating one or the other from the Arduino. You MUST isolate one of the two, or both.
As you stated, the grounds of the solar panel and the battery are isolated, so one of your ADC’s must be, as well. That is also a reason why you could not use a single ADS1115 - the different channels are not internally isolated from each other - they all use the same ground reference and have a maximum common-mode voltage of 5.5V.
Your current sensors, on the other hand are not a problem, as they are galvanically isolated from the current they are measuring.