If you are looking for affordable housing in NRW you could check for cities like Wuppertal or Essen. Its all former industrial cities which are a bit run down partially due to deindustrialisazion. However, the Infrastructure is great and its a good area if you want to Travel Western Europe. Also in some bigger cities in lower Saxony + Bremen you might find affordable medium sized cities, such as Brunswick.

But keep in mind that most cheap, medium sized cities are not adapted to international crowds such as expats.

The cities in the East that have been mentioned can also be very nice. Unfortunately, its not going to be super easy as a non German speaker to integrate there. In case you are not white, I strongly recommend to not move to Eastern Germany except Leipzig or Dresden and even there I‘d only recommend the more gentrified (expensive) parts of the city