Is there a word/expression that would correspond to something like "self-distance"?

You should consider words such as modesty and humility. These are both seen as being positive traits but could be taken to an extreme where a person doesn't take appropriate credit for their accomplishments or abilities.

Another word to consider would be objectivity. This captures the sense of lack of egotism, but it isn't necessarily understood to be reflexive.

self-possession

Merriam-Webster: control of one's emotions or reactions especially when under stress : presence of mind, composure

Oxford: the state or feeling of being calm, confident, and in control of one’s feelings; composure: an air of self-possession

FreeDictionary: Full command of one's faculties, feelings, and behavior. See Synonyms at confidence: ... Self-possession implies composure arising from control over one's own reactions: "In life courtesy and self-possession . . . are the sensible impressions of the free mind, for both arise . . . from never being swept away, whatever the emotion, into confusion or dullness" (William Butler Yeats).

Although less literal a match than the other answers, I think you may want to consider this term for the purpose you intended, or the person you intended. It seems självdistans emphasizes inner perspective while self-possession is about inner behavior, but they may converge on the same character trait: not being ruled by one's own emotions. After all, which emotion is more remarkable to be free of than self love? I wonder if this distinction may reflect culture, Swedish emphasizing self-image, English self-control. Almost thought versus action.

Whomever deserves this description in your book, I like them already.

The problem here is that although there's a Swedish word that can be used in certain contexts, I suspect it's quite likely different Swedes would interpret even that word differently.

For example, a person might be said to have the självdistans attribute if he acts in an objective manner (because he's deliberately being equitable, evenhanded, impartial, impersonal). These meanings all tend towards a general concept of fairness.

But another Swede may say a person has the självdistans attribute if they lack a strong sense of personal identity. Clinically speaking, people with autism/Asperger's syndrome fall into that category (the link says they 'lack brain signal linked to sense of self'). That's an extreme case, but there are a whole host of words in that general area (egoless, detached, unengaged, etc.).

There are also plenty of words involving spirituality, serenity, unworldliness, etc.

And if a person lacks the ability to protect their own interests, they're naive, gullible, artless, etc.

All such words will come with connotations, but probably OP's own objectivity would be the best word for most contexts, if he wants to minimise positive/negative associations.

Balanced ego might fit your need. As the phrase suggests, a person with a balanced ego has one that is optimum, neither too strong (bad) nor too weak (also bad), but in the middle (good).

A similar phrase is balanced personality.