I have a speculation regarding Brook's Flashback and what it contributes to the Elbaph arc. We were introduced to the Domi Reversi and Gorosei control in the Elbaph arc and Rocks' flashback. Now, you might be wondering, why didn't Imu control St. Saturn back in Egghead? Because it was relevant now. The scene of Imu controlling Saturn and Rocks in the flashback provided context for Gunko being controlled, in the same way as Domi Reversi in the present provides context for Rocks and probably Brook's flashbacks.

I speculate that Brook has witnessed DR in his past life. We might see another Native Hunt in Esperia in the flashback. Based on King Ruven's design, he doesn't look like a purely kind-hearted character, hence Brook's minor antagonism towards him. He will probably sell out Esperia to the CDs, Candel will get DR'd, Brook will have to kill her, which would lead to him becoming a pirate, adding to his tragic backstory.

Of note is also him singing Binks' Sake, and having an almost Nika like pose in this chapter. Not sure how that would add on to all of this.

EDIT: After reading some convincing comments, I think Gunko is the one who gets DR'd, murders Ruwen, and Candel falsely accuses Brook of the murder, or maybe to protect her daughter's actions, leading to Brook being kicked out of the Battle Convoy, again loosely implied by dialogue in this chapter, leading him to become a Pirate, and his "Patricidal Princess" line a few chapters earlier. The Nika Pose and Binks' Sake bit of this chapter could hint at how Joyboy became a pirate in the first place.

A bit of Speculation regarding the Brook Flashback.