> A huge hole roughly opposite...

Sounds like the Martian mantle is (or was) organized as a single giant convection cell, with the upwelling under the volcanoes and sinking at Hellis Planitia.

Effects of 7km upon the atmosphere seem easily predictable. Ditto the benefits of 7km for exploration - improved aerobraking, drone performance, etc. But not so predictable would be effects on the subsurface.

An obvious downside of 7km would be "better" air cooling of equipment that needs to stay warm during Martian nights.