Need advice on really long fishless cycle. The problem - lack of nitrates

Actually nitrate is harmless. Unlike nitrite and ammonia.

Even at high concentration, it will not directly harm the fish. Only in the long run, super high nitrite (something like 100~150ppm or even higher) will lower the immune system of the fish. Generally, we keep it at 40ppm or lower. 20ppm or lower for more fragile fish. Of course, the lower the better, but there is no immediate danger with nitrate.

Your plants see nitrate as food. If they are not doing well, it can be other reasons. Such as they are not getting adequate lighting, or not enough other nutrients. What kind of lights and plant substrate are you using?