If the “Switched 3v3” or “Switched 5v” is turned off, then nothing connected to those pins will be drawing power. But if your Mayfly board is not actually going into deep sleep, then it’s sitting in idle mode instead of actually sleeping, and that’s why it’s drawing 6ma. Wasn’t there some recent threads about how the I2C device was keeping the board from sleeping properly?
What’s the reason for having a separate ADC board? Are you just worried about reading a voltage greater than 3.3v with your Mayfly onboard ADC? Because if that’s the only reason, then it would be easier to just put a resistor divider on the sensor output and connect the center of it to your Mayfly ‘sADS1115. If you pick 2 perfectly matched resistors (like 10k) then you simply double the ADS reading to give you the actual sensor voltage.