Importing large amounts of data

A little backstory to this question: I'm working on an (mmo)rpg which has a decent amount of items and skills and other stuff. I have separated the data classes from the gameobjects (meaning that monobehaviour is only on the weapon model and item model etc.)

All the data classes use polymorphism and have a lot of subtypes. Below is an ERD showing how I envisioned the database. https://www.dropbox.com/s/hytej98aih2j927/erd.jpg?dl=0 (Subtypes I am referring to are Entities derived from "Item", "Skill", and "StatusEffect". Everything else is data that can change at runtime (player save data)).

I have minimized the amount of classes needed to get these entities. (mind you this is not 100% correct and doesn't have all the data just yet). It does give an impression of what I'm trying to achieve though.

For the localized version, I'm using a few databases. This is essentially a class with a static getter and a dictionary for the items in the database. I'm also using scriptableobjects to manage gameobjects (such as weapon models).

Based on this imformation I have 2 questions: How do I enter every weapon model and other models into these databases without too much tedious work (and still have a decent overview of what model matches with what ID).

How do I enter all my other data into the respective databases? Some items have information across multiple databases (referenced by an (int)ID). Again, without too much tedious work and still keeping a nice overview of the data.

I could enter the data via a custom editor although I am unfamiliar with this. I would need so adjust the editor for every subtype as well, plus this wouldn't give me feedback of what is in the list already.
I could also use regular C# in Visual Studio or Unity to convert the subtypes to a JSON-like string and convert add them to the database in Unity.
Another option is to already make a database and extract + serialize the data from there.

What would you suggest?
A big thanks for the long read and help in this matter!