Speculations Harald became Garling’s slave

[Monkey D Theories]

This would make a lot of sense, as it would allow Oda to club Shanks’s backstory with Harald’s.

GV also focused on the backstories for Teach and Dragon. Oda could further club those into the Harald flashback as well if Harald was Garling’s slave.

Perhaps Garling pursued Eris and young Teach. Maybe he tried going after the revolutionaries as well.

Lastly, Loki said the holy knights could be defeated, implying he had some prior experience with them. Perhaps he fought Garling at some point?

 

[Blahmigooo]

This would make a lot of sense, as it would allow Oda to club Shanks’s backstory with Harald’s.

GV also focused on the backstories for Teach and Dragon. Oda could further club those into the Harald flashback as well if Harald was Garling’s slave.

Perhaps Garling pursued Eris and young Teach. Maybe he tried going after the revolutionaries as well.

Lastly, Loki said the holy knights could be defeated, implying he had some prior experience with them. Perhaps he fought Garling at some point?

Until last chapter, I'll probably think you write this while dozing off again.
But you might be right as per canon

 

[Worthy Man]

I believe that Harald's phrase in the last chapter about "becoming a slave" is more to create an impact, showing that he's willing to do more than necessary for his people to live in harmony with the world. If Oda makes Harald a real slave, humiliated by the nobles and living a period of his life as a nobody, that would be very heavy to read and wouldn't add much to the story, it would only fuel even more hatred from the fandom against the Celestial Dragons/Imu.