Roger never found the One Piece - Davy Jones will deliver it to Laugh Tale very soon (1138 spoilers mentioned)
The Final stand between Imu and Joyboy:
During the void century, as we see in the mural Joyboy, Emet and his alliance challenged Imu to a final fight. We see in the mural that Imu seems to have the eternal flame, the power source for the ancient weapons. However we know he no longer has the flame because he needed Vegapunk to recreate a facsimile to power Uranus.
This fight happened in the ocean by the redline, right over fishman island. We know this because after being defeated and falling into the sea - Emet awakened 600 years later under Marijoa.
The original Vander Decken, who was an ancient pirate from the void century, was also cursed by a God and he and his crew fell into the ocean above fishman island. They were allied to Joyboy and fell in the same spot as Emet because they were also cursed by Imu.
So we know that Imu can dish out curses to his enemies causing them to be cursed by the sea. We actually know one other ‘mythological’ character that fits this description. Davy Jones
The sea current and the cursed Pirate Davy Jones:
The pirate Davy Jones was said to be cursed to wander the sea floor eternally. Being an ancient pirate, he seems to be another victim of Imu’s curse from the final war. Except instead of staying in fishman island he wanders the sea floor collecting great treasure that he has found.
The great underwater sea currents are said to take up to ~2000 years to surface at maximum. So if someone were to start their journey in the sea current near the halfway point it would take them a little under a thousand years to resurface? So a cursed ship traveling from fishman island may take around ~800-900 years to resurface at the end of the grand line.
I believe the cursed Pirate Davy Jones was an ally of Joyboy, that after being cursed by Imu to wander the seas, took a great treasure that was dropped on the sea floor during the battle and will surface with it at the current day.
Binks Booze and Brook:
The song of delivering Binks Sake and Brook being a man who lost his crew and was forced to wander for 50 years on a ghost ship until Luffy found him is a parallel to Davy Jones crew dying and waiting 800+ years to deliver the one piece.
Roger named the final island ‘laugh tale’ after the end of Bink’s Sake, because Binks sake is about Davy Jones delivering the one piece to laugh tale. Read the song with this in mind ‘Never Ending Ever wandering our funny traveling tale’.
Roger did not find the one piece:
Rayleigh mentions ‘there is nothing any of you could do about it now’
Whitebeard tells Sengoku ‘when that thing is found the world will be turned upside down’. Implying that Roger didn’t find it.
Roger tells his crew that they were too early when arriving on laughtale.
Toki knew that Joyboy would appear 20 years in the future- the specific year that Davy Jones ship would arrive at laughtale as told on the poneglyphs. That’s why she chose that time to send Momo forward to.
The reason that Roger and his crew laughed when arriving on laughtale is that there was no treasure, not for them. Gaimon’s story is a parallel to this.
Roger gave himself up in part to inspire the next Joyboy, getting as many adventurous people to look for laugh tale as possible.
What really is the one piece:
The one piece is the eternal flame , dropped by Imu into the ocean during the climatic battle with Joyboy. Joyboy and his allies were defeated in this battle (several of them like Decken, Jones and Zunesha were also cursed), but their small victory of getting the eternal flame away from imu saved the world from being completely destroyed.
The poneglyphs, and the final bit of the true history on laugh tale tell the story of Joboy and his crew, joyful - adventurous pirates like the straw hats. They also tell about how the power source should be used to power the ancient weapons and destroy the redline.
The two sovereigns mentioned by the sea kings are shirahoshi and Luffy. When they meet again with the Eternal flame and use the ancient weapons the sea will be open again, the world will be connected back in one piece.
The sea kings mention that ‘the whales will be delighted’ because the natural migratory paths of the whales have been disrupted by the redline , that’s why so many have scars in their heads from bashing the redline. The sovereigns meeting will delight the whales because their migration patterns will be fixed.
The lines in the Harley about ‘they will never meet again’ is about the oceans. At the end of the first age the redline divides humanity in two. At the end of the second age the grand line divides them further. At the end of the third age these barriers will be removed and the oceans will surely meet again.