It's interesting how First 6 SHs were chosen,
Only Island that was only visited by these 6 was Drum Island
I've been saying countless times that Drum Island is a Mini-Version of Void Century with Hiriluk being Joy Boy & 20 Isshi Doctors being 20 OG Kings & both Kingdoms were "Nameless" until a Pirate Flag gave it Purpose, Hiriluk's Dream was also a Re-Skin of Joy Boy's (Just replace Snow with Laughter, hence why their Dying Flashbacks were called Snowy/Laugh Tale)
This is also the Perfect Opportunity to explain why Blackbeard sought that Island, and it's no coincidence that We have an Unfinished Sub-Plot regarding Wapol/Vivi (Former King of Drum Island + Another Semi-SH who Visited it)
Another thing that People do not realize about Drum Island is that it's First Island that SHs reached because of a Storm, it's like They were Fated to reach it, unlike previous Islands which just happened to be in their Path or were guided by Someone or They wanted to go there
Second Island where similar Scenario happened is Thriller Bark, where a Storm led them to meet Brook, something that Roger Pirates failed to do for long time, showing that SHs can only be guided & it was thanks to that Storm
So yeah, this Separation imo further confirms to me that Drum Island Arc is Mini Version of Void Century/WG Arc, while Thriller Bark is Mini Version of Ending Arc against whoever is controlling events (Someone who controls the World in a Literal way, not Politically like Imu is doing)