I think height is what you are missing. Something that would start at the bottom of the aquarium and then continue up to the surface. There are a few ways you can provide that.
Taller stands of dense plants toward the back of the aquarium would work well. Water Sprite, Wisteria, or Crytpocryne spiralis are good options. Many stem plants would also do, such as Ludwigia. There are a lot of options for background plants for such a small tank, the main concern is making sure they will tolerate warm water since it is a Betta tank. As far as bamboo goes - Lucky Bamboo is actually not a true aquatic, and if you put it underwater it will eventually rot and make the water quality dangerously bad. There are quite a few fake bamboo decorations available, though, if you like that look. Some people grow bamboo by keeping the tank without a lid and allowing the majority of the plant and all the leaves to grow above water, but since Bettas are a) jumpers and b) do best with a tight lid with no gaps so that they can have access to humid air for their labyrinth organ, I wouldn't recommend trying that sort of setup.
A branching piece of aquatic spider wood that's tall enough to nearly reach the surface of the tank could also work. If you don't want to boil it (I honestly just use my regular pots), you can soak the wood in a rubbermaid bought for the purpose so that most of the tannins are released already by the time you add the wood to the main tank. Soaking may take a week or two, though, whereas boiling is just an hours long process. But in a tank as small as 5 gallons, there are a lot of hardscape options that would work, including some larger resin or ceramic aquarium ornaments. I suggest you measure how much space on the tank you would have from substrate to just below the surface with a tape measure and then go to your local pet stores to see what all they have that would be around the right size. The pet stores I go to have never made a comment even if I bring my tape measure and hold it near decor to see how close it is to the size I want.
I also agree with the others that a background would be nice. You could choose a black one to contrast with the other colors in the tank and add a sense of depth. Or you could use one patterned to look like wood, rocks, or plants to give the eye the impression that there actually are taller things in the back of the tank already.