How to make JSON.stringify to only serialize TypeScript getters?

I have the following class structure:

export abstract class PersonBase {
    public toJSON(): string {
        let obj = Object.assign(this);
        let keys = Object.keys(this.constructor.prototype);
        obj.toJSON = undefined;
        return JSON.stringify(obj, keys);
    }
}

export class Person extends PersonBase {

    private readonly _firstName: string;
    private readonly _lastName: string;

    public constructor(firstName: string, lastName: string) {
        this._firstName = firstName;
        this._lastName = lastName;
    }

    public get first_name(): string {
        return this._firstName;
    }

    public get last_name(): string {
        return this._lastName;
    }
}

export class DetailPerson extends Person {

    private _address: string;

    public constructor(firstName: string, lastName: string) {
        super(firstName, lastName);
    }

    public get address(): string {
        return this._address;
    }
     
    public set address(addy: string) {
        this._address = addy;
    }
}

I am trying to get toJSON to output all the getters (excluding private properties) from the full object hierarchy.

So if I have a DetailPerson instance and I call the toJSON method, I want to see the following output:

{
   "address": "Some Address",
   "first_name": "My first name",
   "last_name": "My last name"
}

I used one of the solutions from this Q&A but it doesn't solve my particular use case - I am not getting all the getters in the output.

What do I need to change here to get the result I am looking for?