Mention has not been made of the plant species, and this is an important factor. Adding excess nutrients (more than absolutely necessary for the plants) is detrimental to fish, and the biological system. One of the advantages of good substrate fertilizer such as Seachem's Flourish Tabs is that they release nutrients as they are taken up by the plants, and excess doe not get into the upper water column. Several years ago upon learning this, and having nutrient-lacking tap water (zero GH), I reduced my liquid fertilizing and added the tabs, and the plants took off. Large substrate-rooted plants (swords, bulb-type, etc) especially benefit from this. I was able to discontinue using a calcium/magnesium fertilizer (Equilibrium) when I added the tabs, replaced every 2-3 months. You obviously won't have the hard mineral lacking problem with your water, but the tabs really do make a huge difference to such plants. And the fish are not impacted, a true benefit.
You asked about DIY substrate tabs. This is not advisable. Aquatic plants have a different nutrient requirement than terrestrial plants, especially in the area of nitrate and phosphorus, and every DIY I have seen seems based on products like Osmocote. This is not safe in a fish tank.