No, 1.4 wpg from T-5 HO is a lot of light. That much calls for pressurized CO2 on a tank that big.
5 ppm NO3 is the absolute lowest that I would let it get. Maintain the dosing for a week or so, and see how much the plants are using. Dose more as needed. In my low tech tanks I allow it to drop to 5 ppm because I know it is not going to hit 0 ppm too fast. In a high tech tank the plants can use 5 ppm in just a day.
Yes, P tests can be inaccurate, and there can still be enough P for the plants that are too low for the tests (hobby kits) to show. If you still want to use Phosorb use it when you are pre-treating the water for a water change. Do not allow it to hit 0 ppm, though. Plants seem to do well at any level from under 1 ppm to 5 or more ppm without triggering algae growth as long as other issues (like carbon) are under control.
Having rock that adds minerals to the water is helpful, but when you do a water change it takes a while to build up to that mineral level again. The tank goes through swings in mineral levels that are not good for the fish. Best is to alter the water before the water change, and use the rock as a way to fine tune the water between water changes.
A TDS meter might be of benefit here. When the tap water is questionable, or known to vary, this can give you more info about how variable it is.