Hello there!
Since nobody from Aachen seems to be on here, I´ll try to give you a long-distance estimate as a wife of somebody who part of a visible minority.
In general, I would say, the bigger the city, the better. It pains me to say but I´d also stick to West-German places (with exceptions and that does not mean that everybody born and raised in the former western part is tolerant and pleasant towards minorities nor that all East Germans are xenophobic. I just know that there are places where I will not go with my husband, because we feel uncomfortable). As Aachen is as far west as you can go within Germany, no worries there, lol.
Aachen is not really big but I think big enough. It has a university, so there should be a fair amount of international students. It is also in an enviable location, bordering on the Benelux countries, which is great for travelling and -again- generally speaking, towns close to the borders of our neighbouring countries tend to be more open and accepting due to a long history of trade and exchange with other cultures.
I visited once and found it lovely.
Is there a possibility to spend a weekend in Aachen to see how you feel there?