I feel like we could have fixed it with modern science...

You cant un-break the ship's "spine". Its made of wood and no matter how you reinforce it with the tools available at the time, the stress would just eventually break it again and sink it.
The Galley La shipwrites even told them they could replace the keel but at that point it isnt even the Merry anymore, it just looks the same.
Also look up the ship of theseus, the thought experiment is kind of on the same vein as this issue.

Shipwrights*
Kms.

>the moon had ultra high tech and everything ran on electricity
when was this stated? I only watch the anime fully caught up on sub, but shit prior to time skip is spotty memory wise.

Enel had a series of cover stories about his adventures on the moon, that the anime never adapted.

onepiece.fandom.com/wiki/Enel's_Great_Space_Operations

Should have fed the ship a devil fruit so it could grow and heal itself

They can’t rebuilt it, they don’t have the technology
Worse
Weaker
Slower

This, if you replace the broken part it's not the same boat.

Because Oda is a hack and had to do it for melodrama

Admit it. You still cried.