Oda focused on slavery as one of the themes of Wano by putting Luffy in the labor camp. Like many of the previous arcs, there's an organization or government (the Beast Pirates and Orochi) who are oppressing the masses, and Labor camps are one method of oppressing the people of Wano
Oda just showed us broken samurais and Luffy saving them not Slavery.
I never called everything a theme. I merely stated that the reader may interpret multiple themes, despite the author intending more or less. Again, you are arbitrarily stating that One Piece doesn't have as many arcs as we interpret, despite that interpretation of themes being completely subjective.
Interpretation and themes are different concepts, sure. But how exactly do you locate themes present within a story? Through interpretation of the story.
Again, if a reader interprets a story 20 different ways, who is to say they're wrong? Ultimately, interpretation is completely subjective with no right and wrong answer.

The message of slavery within Wano is to strive for freedom by not giving up hope - as seen in Udon.
Thus, slavery is a theme within Wano.
Thus, slavery is a theme within Wano.
Even your interpretion doesnt have Slavery as a theme but rather Hope as a theme.