Hairgrass is not an easy plant, and I will leave that for more experienced members. I have heavily planted tanks, but my light is deliberately moderate to low so I avoid these difficult plants which I have tried without success.
One thing I would suggest though is floating plants. Betta naturally cruise among floating vegetation, and will feel less stressed if floating plants are present. This will obviously reduce the light getting to the substrate, but I am a fan of providing more what the fish needs than plants.
Byron.