I think that both internal and external sources wouldn't be fair about the data in two opposite ways, either improving or destroying the image of the country.
What could be a trustable source of information, if there is any?
A problem with giving the DPRK a "fair" assessment is that such an assessment would be based on value judgements, and the DPRK does not share the same system of values as most of the rest of the world. You are right that many sources are biased by their historical opposition against the DPRK, or consist of recent defectors who thought they had good reasons to defect.
- Look what the United Nations Human Rights Council had to say. Of course the UN is highly politicized, but it is also often deadlocked by competing interests.
- You might consult NGOs which regularly criticize western governments as well, and see what they have to say about North Korea (AI, HRW). Of course this criticism is still based on certain western value sets, which western governments fail to live up to, and which the DPRK outright rejects.
- If you are prepared to trust the translations, read the 10 Principles .
- A possibility might be to look at Chinese sources about the DPRK. China was an ally of North Korea, and like North Korea it has a Communist-Party-based government (each with their own development from that baseline). My problem, I only get those in translation.
I think that one of the best places to look for information about the North Korean political and social system is to defectors. Tens of thousands of defectors live outside of North Korea, especially in South Korea and China. It is also a good idea to gain an understanding of their unique communist system. In other communist countries, we do not see the level of diefication we see of the Kim family in North Korea. The Kim family has used the sickle and hammer more as an icon of the family than for the party.