is only reason to aerate...tank location, vent lines that are too long, too convoluted or vertical also make passive ventilation impossible.
As for a separate flush water tank, people always just LUV to hear that there was a better way to do something AFTER they've already done it. But there is: simply tee the head
intake line into the head sink drain line just before the thru-hull. To flush with fresh water, close the the thru-hull, fill the sink with clean water (don't use gray water...it's not good for toilet pumps)...flush. This also allows you to flush with sea water, providing a safe way to supply fresh water to rinse it out of the WHOLE system--intake line, pump, channel in the rim of the bow AND the discharge line.
You DO realize that your flush water tank needs to be totally separate from your fresh water plumbing--separate fill, separate vent...NO connection to the fresh water system at all. If you need to correct that, the easiest way to do it would be to tee the fill into the sink drain line.