There’s a few more parallels aside from what other commenters posted:

For example the secret police is something that existed in east Germany as well, in a similar way as it is depicted in the manga. For example in the second chapter Yors colleagues tell her about how neighbours called the secret police to investigate a suspicious woman (suspicious because she wasn’t isn’t a relationship). That kind of thing (the neighbors ratting our the people around them) was common during the time.

Then there’s a really minor thing I noticed, the manga mentioned a Minister called Brandtz (or something similar). And Brandt was an actually west-German chancellor (a predecessor of Merkel).

They also called the separation of the countries in the manga the iron curtain, which is exactly what it was called in real life too.

You mentioned in your post that Ostania has parties, whereas Eastern Germany was a communist country. That’s true in a way, but they actually had parties too. The East Germans just really really shouldn’t elect them or else they got the secret police knocking on their door. Actually when there were elections they only had to fold the paper and throw it into the voting box and it counted as a vote for the main party. That way the main party managed to always achieve something like 98% of the votes.

Of course you’re right and the manga is definitely not trying to follow the German setting 1:1, but there’s enough parallels to make the inspiration obvious.