now the one million dolla question how can i achieve not only 0db for all tracks but also more or less same "hearable" loudness for all tracks?
The million dollar answer:
There's no "master" button. Rely on your ears, not on the meters. 0dB means nothing, unless you're absolutely in the final stage of mixing and mastering, which you're apparently not.
Or, like it's been said already: take it to a mastering studio.
*edit: By the way, it takes YEARS and YEARS for mix engineers (who've already been doing it for years and years) to get to the stage where they can successfully master an entire album the right way. If it doesn't sound good to you, work on it, but just know that it might take you your whole life to get it right. Long as you're okay with that...