A 6500k light is perfect for growing your plants.
I have floaters in all my tanks and just put them towards the front or off to the side so they don't block the entire top and it seems to work. I keep them trimmed so they don't get to big.
I also dislike hornwort, it's rough and itchy and is hard on a bettas fins. There are tropical hornwort kinds that are supposedly softer and not as hard but touching the hornwort we have just creeps me out and often leaves red scratches on my hand when cleaning the tank.
Water Sprite is wonderful. It's beautiful and very easy to grown. You can float it or plant it. We have one mother plant that is planted in my 29g and all the other babies that float came from her. The planted one is so, so very big and the roots are throughout at least half of the aquarium, so I imagine it sucks up a lot and it slows the water flow down, which is nice for a betta.
I've not tried DHG yet in our tanks but plan to soon. With your 6500 light, you should be fine so long as the light reaches the bottom. I've read so many differing stories of DHG, some people grow it well in medium to low light and some people don't. Probably just depends on your tank. I plan to get a small bag or two of it next time we go to Petco to try out.