The water pressure at the top of the tank is insignificant. How much water pressure is exerted by an inch of water?

Also, if the tank is properly vented there is zero pressure from pumping. It is all vented out as the air is replaced with liquid.

If the tank should happen to fill to overflow, the column of water going up the vent hose and overflowing into the ocean would add pressure as the vent hose would usually be smaller than the 1.5 inch hose from the toilet.

Once outside the U.S., until Tom gets to NZ or Australia, by his design the tank will never fill. I don't care for this design idea because the heeled water line may be above the bottom of the tank and there will be flowback and sea water sloshing around in the tank. The conventional Y valve before the tank is a better choice. When in a marina in NZ you will not be allowed direct discharge - actually anytime you are within some distance from shore you are not allowed direct discharge, However, I have never seen a pumpout station in NZ.