Still contradicts his character as being the most free man in the world, when his whole life was determined 800 years ago by prophecy
Well....this is a concept we face in our life itself.... it's not bad story writing.... it's TRUE LIFE...
The whole existing of us human beings.... whether religions are actually legit...whether a higher being actually is real... and if so... are our lives determined from the get go? if not... and we are being tested and there is heaven and hell... then, couldn't god speed it up since he can see everything including the future???
This is some real life shit, a paradoxical dilimma... So,yeah... Luffy can feel he's most free person... while his life course was determined long ago....
Point is... it's very okay storywise... and it's not bad to have some paradoxical thing that exist in our life itself