You'll find the subject of Juwel filters is one of controversy. Some people love them, others hate them.
Generally speaking, you need a turnover of 5 times the tank volume per hour. But I'm sure I read somewhere that Juwel's idea is that a slower water flow means the water, with its ammonia and nitrite, is in contact with the media for that little bit longer on its way through the filter. The built in filter is OK if you don't stock the tank to the max, and if you avoid messy fish. Algae eating fish (eg plecs) and some big cichlids are the prime examples of messy fish, though you won't be able to have the likes of an oscar with 180 litres.
I have a Rio 125 and I removed the built in filter, but my main reason was the noise. I've never heard of anyone else with this problem, I even tried a new powerhead to no avail.
Yes, you could try a more powerful powerhead, just make sure it'll fit. Or you could get another filter - an external would be best for a 180. As you are not far into your cycle, you would only be losing a few days if you chose to get another filter. Or even get another internal to run with the built in filter, though that would take up more room in the tank.
If you decide to keep the built in filter, even with a bigger powerhead, just a word about the media. In mine (I've had it a few years) there were coarse and fine blue sponges, a green sponge, a black sponge and a whilte pad. You'll most likely also have a box of cirax which was brought in after I got my tank. Ignore everything the manual says about changing the media
Cirax needs to be changed in a few years' time when it starts to crumble; the blue sponges need to be changed in several years' time when they start to fall apart. Until then, squeeze the sponges gently and swoosh the cirax box in old tank water that you take out during a water change. You only need to remove the debris off the sponges, so don't overdo it; they'll never look like new again.
The green sponge (if you have one) isn't very good at removing nitrate like they say. But it is a good home for the filter bacteria, treat it like a blue one and don't change it.
The black sponge has carbon in it. You don't actually need carbon. It will grow bacteria in it, but carbon gets used up very quickly so this is one sponge that is supposed to be changed frequently - and the cost will build up. At this stage in a cycle, there won't be many bacteria in it. I would take it out and replace it with another blue sponge - or leave the space empty in case you ever do need carbon. Its main uses are removing the brown colour that leaches out of bogwood and removing medication after treatment has finished.
The white pad - this will need changing frequently. Unlike most filters, Juwel puts the white pad at the start of the water flow to stop debris getting into the sponges. It will clog quickly and won't go back to shape after washing. I only managed one wash per pad while I was using the built in filter. Buy a roll of filter wool (pond stuff is cheapest) and cut it to shape.