>the biggest challenge is that the logic for mutating that object is all over the codebase
Just use immutable data structures and be done with it. In departing from old OOP views and becoming more functional programming and data oriented programming friendly, C# introduced Records, which are immutable. Probably Java and Python have similar constructs. Javascript allowed the use of immutable data since long time ago.
If you insist of using fat models, you will still mutate the data all over the place by doing calls, but you just obfuscate it.
> Probably Java and Python have similar constructs
In Python, the closest you can get is a "frozen" dataclass, but you don't get true immutability[0]. What you _do_ get is effective enough for just about all practical use cases, except for the standard foot guns around mutable data structures.
You can redefine the byte representation `True` corresponds to in python. "Immutable enough" is all you're really looking for; it somebody goes out of their way to mutate the thing then they probably had a good reason for it.
Just use immutable data structures and be done with it. In departing from old OOP views and becoming more functional programming and data oriented programming friendly, C# introduced Records, which are immutable. Probably Java and Python have similar constructs. Javascript allowed the use of immutable data since long time ago.
If you insist of using fat models, you will still mutate the data all over the place by doing calls, but you just obfuscate it.