Object-oriented design: what are some easy-to-use tools for prototyping conceptual data models?

I am in the initial stages of planning a potentially large project, and I am looking to create prototypes of the basic data structures involved before getting stuck in the coding of all behavior / logic.

Ideally, I am looking for a GUI application with a declarative interface that allows me to define arbitrary data types, specify hierarchical relationships between them, store instances / records of those custom types and export these models / data sets in a tabular / serialized file formats .

I have considered writing JSON or YAML in a text editor or using a spreadsheet application to approximate a relational database, but none of the approaches really do a great job of capturing / visualizing relationships between objects. That makes sense considering that the serialization of data is the point of JSON and YAML, and a drawback of relational databases is that the associative tables are much less readable than simply having a nested / embedded list in the object's record. principal.

I am aware that there are many conceptual / mental mapping tools, but as far as I can tell, they tend to be less structured than I am looking for as a general rule. Is it time to choose a suitable modeling language like UML / SysML, or is it exaggerated if I am not looking to model any behavior? The only thing that comes closest to me is like a kind of wiki in which you can define templates for different types of records and link between records / instances of those types, but obviously I don't want to mess with all the HTML headaches if I don't I need it.

Obviously, my code will eventually be the real representation of this data model, but is there a quick and convenient visual tool for prototyping these kinds of things that I am missing?