SlowButSteady is right about the waste in the line and it will eventually permeate the hose. Even if the waste line does not defy gravity, it requires at least one full gallon of clean water to keep it clear of deposited particles. One way to fix this would be to get a new hose and route it from the head, straight up along side the hull to a point that is higher than the waterline when heeled, install a vented elbow to prevent a siphon and then route it any way from there to the holding tank without ever creating a low spot. The only part of the hose that would have standing water would be the piece from the head to the vent and that should have clear water, if you flush enough. You can probably get away with the hose you have routed up to this new vent and only buy the hose to get back down to the head.
The problem with a 6 gallon tank is it is legitimately about 5 to 6 flushes. They really need to be vented outside the boat, which requires a 1" hose, also run with an anti-siphon loop, to a thru hull above the water line at heel or through the deck. While not required, I would hope it has a way of emptying overboard, when allowable.
If you were willing to spend some money, the Lavac Head may be the answer for you, if it will fit. They work by hand pumping a vacuum that builds up and then shoots the contents of the bowl to the tank. You would probably still need to re-route the waste lines away from that dip, but they only use a few pints of water per use and your 6 gallon tank would last much, much longer.
Good luck. For me, any boat without an operating head can not be cruised and isn't even a day sailor, its a couple of hour sailor, maybe.......