How to translate the German term "Selbstverständnis" with respect to organisations?

The German term "Selbstverständnis" can be used in the context of (typically) not-for-profit / non-profit organisations to denote the aims they have and the (typically social) changes they try to achieve. It typically describes their ideology and philosophy as well as usually the way the organisation thinks it is best to succeed in reaching their goals. It gives you an impression of how the organisation works and what sort of people their members are.

Is there a word that conveys all these nuances?

Selbstverständnis literally means 'self-image' or 'conception of oneself', so I think your own use of ideology fits well:

the integrated assertions, theories and aims that constitute a sociopolitical program M-W definition

You might also see this referred to as a Mission, or mission statement, even though it is not really a direct translation.