From my experience, you have enough flow. I have the same filter on my 72g tank (4 feet width) without any issues. You really just want very good biofiltration that will eventually make the tank work. Right now even if you add a bigger filter the biofilter isn't mature so your just moving water around. I also had an iwagumi on my 46g tank (3 feet width) and used a Eheim 2213 and it was pristine. Once tank develops most of the filtration will be in the tank anyway. That's why initally you do the water changes and use carbon and/or Purigen to help with organic removal since the plant mass is limited. Once the plants get going, you remove the carbon. Adding stems as suggested should help as well.