First thing I changed on our new XP 2.0 ST bikes was to change the freewheel from 14-28 to 11-34. This gives a considerably-lower first gear to help with climbing hills.
As for power, I note that on the hills around us my bike pulls 18 amps at 48v at full-throttle. That's about 900 watts, or a bit over one horsepower. The fact that the motor is rated at 500 watts just means that it can't pull those 900 watts forever or it will eventually overheat, but then around here I'm at the top of the hill in maybe five minutes.
I'm still learning about this stuff but from what I can tell the 48v/18a parameters are a limitation of the battery pack, not the motor. The lithium cells in the pack are 2600mAH, times four in parallel gives you 10.4AH. If the cells are rated at "2C" I understand that means they can reliably be discharged at twice that, or 21 amps.
From what I'm learning, if you want stump-puller ability then you might want to look at "mid-drive" bikes, where the motor is at the crank and can take great advantage of a very low sprocket gearing. But perhaps your quickest and cheapest option is to first change the freewheel to aid in your climbs.
