If you're sure you're never gonna flush the toilet directly overboard, the easiest solution would be to eliminate the hose from the toilet to the thru-hull. That would not only eliminate the need for a vented loop in that that line, it would also eliminate the y-valve that provides a choice of flushing overboard or into the tank.
However, unless you mount the macerator pump above the tank, you will need a vented loop in the line between the macerator and the thru-hull.
Btw...if your holding tank vents into a rail stanchion--which it does on most Catalinas more than a few years old--this would be an excellent time to reroute it to a thru-hull...which you'd have to install. It can go below the toe rail directly below that stanchion. And it should be a 1" open "bulkhead" thru-hull that you can stick you finger into instead of the typical 5/8" "vent" thru-hull. Several Catalina owners here have done it...there have been a few threads in the last few months.
--Peggie
"If you can't explain it to a six year old, you don't completely understand it yourself." --Albert Einstein