Hi. I see two issues with that bike that would limit its climbing ability:

1) as you mentioned, the gearing is too high. There are two solutions:

a) run a smaller chainring
b) run a larger cog

Most stock IGHs use a low mid high setup. I also live in mountainous terrain and I like to set up my IGH as lowest low mid. In real life terms it means that instead of having a tall gear for downhills, my tallest gear is for flats and then I have two lower gears.

2) The slack seat tube angle and forward bottom bracket take you out of a power position

More gears could be better but three could be enough. It depends on you.

Here is a link to a gear calculator for nexus. It also tells you the different ratios available

Shimano Nexus Internal-Gear and Generator Hubs