Your Pothos looks very healthy. I see only one yellow leaf. A limited number of yellow leaves is inevitable and not a cause for concern or necessarily doing something different. As long as your Pothos is adding more healthy new leaves than losing older ones, you are generally on the right track.
Pothos are hardy indoor plants because they do not need or even like the direct rays of the sun falling on them. Your "down under" north-facing window will provide too much sun unless you cover it with a sheer or move the Pothos back away from the window and beyond the range of the sun's rays.
Your watering is probably about right, but you may be allowing the soil to get just a bit to dry. On the other hand, you will need to allow the soil to dry as much as possible in order to deprive the gnat larvae of the moisture they require
The wonderfully organic soil that you used is terrific for OUTDOOR plants but not recommended for indoor plants. It is a non-sterile mix that comes loaded with potential pest and disease problems. Hence the odor and the fungus gnats. The gnat larvae feed on any decaying organic matter. Keeping the top layer of soil as dry as possible is the best antidote, but if it gets too dry, you will get more leaf yellowing.
If the original rootball is still intact, then you can consider undoing the repotting you did by removing that soil and manure you added and putting the original rootball back into its original pot without any added soil.
If you removed a good portion of the soil from the original rootball, then trying to replace the soil you added may be very traumatic to the roots. In that case, I would try to wait out the gnats and hope they go away without repotting.
For future reference, use a sterile potting mix that consists only of coir, peat moss, and perlite.