Honestly, the only real "deeper" take I can come up with is "maybe the manga should have ended after they reached the ocean".
If we want to end the story there:
- Inside the basement is actually a super Titan serum that can fully awaken and overclock Eren’s Founding Titan abilities, and a note from Grisha saying exactly what it does. We see Eren’s reaction to the note, but we do not see what it says.
- Armin and Eren and co. make it to the sea. It is revealed that the rest of the world had been destroyed by a massive Titan outbreak (perhaps triggered by Grisha some time before the events of the story). The other Titan shifters aren’t soldiers trying to infiltrate Paradis, they’re the last survivors of Marley, human weapons meant to enforce mutually assured destruction, which they are now planning on carrying out.
- This leads up to a final battle between the three shifters against Eren. Except they each also have a trick up their sleeves - serum for the remaining three remaining “special titans” that they had been keeping as a last-resort powerup. The newly powered up Titan shifters handily defeat Eren, at which point he activates his own last resort to save Paradis and injects himself with the super serum from the basement, which initiates a one-way transformation into the Founding Titan.
- With its powers, he screams towards the sky, commanding every single Titan in the world to STOP. The Founding Titan continually broadcasts this psychic wave - cue montage showing Titans in various familiar locations standing up in place and turning to face towards Eren. We see some of them show signs of crystallization at their fingers and toes. This will be accompanied by “visions” of Eren freeing people from their Titan shells.
- As the montage ends, cut back to Eren. His Titan has now taken on a petrified, tree-like form, most of its energy being used to project its psychic signal. Using its tremendous power has left Eren permanently fused with the Founding Titan, but for the sake of plot convenience he’ll be partially ejected from its neck so his friends can have their last words with him before he loses his individuality and fully merges into PATHS, where he’s created a new world for everyone who “died” as a Titan.
- The story ends with everyone who’s still alive alive, everyone who was dead staying dead, and Eren, the other Titan Shifters, and every single person who was in a Titan ending up inside PATHS.
So there, the story would end with Eren achieving his Shonen protagonist goal (killing all the Titans/saving his homeland) and his personal goal (seeing the ocean). By sacrificing himself, he creates the Titan-tree that continuously broadcasts a signal suppressing the Titans and causing them to slowly petrify, and situates himself in a spot where he’ll forever look out towards the sea.
This will likely require rewriting some of the background lore, but should be mostly compatible with the original premise. I don’t think you can end the story at the ocean without also addressing the whole Titan problem, because that would leave the entire premise of the story unsolved. And giving Eren a massive powerup that requires him to sacrifice himself to use seems just about the only way to really deal with it.