Do not lock up phosphates with filter inserts. Lock it up with plants.
Are you using any source of carbon? Even with only 2 bulbs, T-5HO bulbs, are quite a bit of light, and the plants will need more nutrients, including a source of carbon.
Many of the plants we use in our tanks can use the carbon from carbonates, but not all.
KH 50-85 mg/l is VERY low for Rift Lake Cichlids. I keep mine closer to 150+ mg/l (at least 9 German degrees of hardness).
GH of 90 mg/l is about half of what I would aim for.
To do water changes in my Lake Tang. tanks I run the water into a garbage can, add minerals and dechlor. then circulate it until the minerals are dissolved. I use baking soda for KH and Seachem Equilibrium or Barr's GH booster for GH.
pH in this tank runs pretty consistently in the highest 7s. GH and KH about 9 German degrees of hardness.
I do not have problems growing plants in hard water. Sure, there are a few plants that really are soft water specific, but I am not trying those in the hard water tanks. Most plants accept a very range of GH, KH and pH with no problem at all.