Now, I don't have 1 large 180+ system .. main DT is 130 total volume, and 2 frag tanks make up another 100G. So total, 280G. I have a deep well, never had an issue/concern about running it dry (esp here in MD). BUT, I did go through a well pump that was only 2 years old. But, I was tapping the water prior to the compression tank, so it was bascially constantly running. Either way, I've stopped making any water. I buy in bulk at a shop near my house.
Related to septic, I would dump no waste water into it. I don't have a "drain field" but septic into main tank, flows into another lage tank (just liquids). Very solid system, but I don't want that much more water going into it.
Related to water changes, I dump none in it either. I know, I know, our bodies have salt, etc .. but, I just don't trust it on my 30 yr old septic.
When I build my big big tank when we add an addition, I will have a drain that I can divert into a pit in the yard, that will be deep, lined with gravel, etc.