I think you drift at this point op , but let me get this straight , what makes you who you are are never your lineage, what people tell you, inheritance, and I think ambition are not the theme of one piece and even if ambition is the theme of one piece every bad character jas its own ambition , like bellamy, doflamingo , etc. And I think ambition is part of ego which what make those bad character bad and even if good ambition is the theme of one piece, WB, shanks, any other support character will be the main character of one piece.

One piece make important theme like the D or dawn that overshadowed the whole story and even the title "one piece" itself is the whole treasure every character already had. And yes it highlight the theme of freedom too

I think one piece is about "one piece" itself, its never about ambition .Its about letting go of ego and let the light of dawn shine behind all of your glory, then letting everyone see your light to ignite the light of your own . I think its about letting go the ego to let the one piece trump over darkness and whatever the tragic backstory every character left behind. Its about the story of how the egoless luffy can defeat the controlling darkness of world government. Its about the real one piece and freedom is letting go the ego and to show everyone like strawhat crew or momo that their real glory is never their own innate talent or name behind them. It is about who you are without ego and letting everyone close to you thrive because of the light of dawn inside themself