Your 55 is very lovely!

It's good you're adding more gravel since the Amazons put out quite large roots and need plenty of substrate. Agree with tetralina that Java fern grows slowly, but will eventually make a lovely and lush bunch. I would tie some to the pieces of wood you have in there. It would look great.

Nice betta too. Does his tank have a lid?

ETA: A very nice foreground plant would be Cryptocorynes!