My 10 gallon betta tank has just a little bit more light than yours, and successfully grows java moss, java fern, and anubius nana. All three of these can be tied to
driftwood/rocks (preferable) or left to float, so substrate doesn't matter. My first betta that was in the tank loved the anubius the best, because it has wide, flat leaves to rest on. The java fern also gives a nice "jungle" effect for the bettas to swim through.
I've also had great luck with amazon frogbit in my 20 gal. betta tank. It's a floating plant but the roots hang down and my female especially loves to swim through them. Just keep in mind that this multiplies VERY quickly so you have to clear it out a bit so your bettas have plenty of access to the surface for air, and also so it doesn't shade out any plants beneath it.
Good luck getting plants! My bettas love their plants and it also makes the tanks look so much better. All of the types I listed are also very low maintenance.