How to correctly pass a JSON from Angular to Spring Boot?

I'm using Angular 12 CLI and Spring Boot 2.5.5. Using an Angular service, I'm trying to pass a JSON from a component to Spring Boot, using a POST http request. After the JSON arrives in Spring Boot, I need to print it in console, but I can't get it to work. Using Postman, I correctly visualize the JSON, so I think the problem is front-end side. Here's some code:

users.service.ts

import { HttpClient } from '@angular/common/http';
import { Injectable } from '@angular/core';
import { HttpHeaders } from '@angular/common/http';

import { Observable } from 'rxjs';
import { User } from 'src/user';

@Injectable({
  providedIn: 'root'
})
export class UsersService {

  constructor(private http: HttpClient) { }
  
  getUsers(): Observable<User[]>{
    return this.http.get<User[]>("http://localhost:8080/user/all")
  }

  addNewUser(user: User): void {

    const httpOptions = { 
      headers: new HttpHeaders(
        { 'Content-Type': 'application/json' }
      ) 
    };

    let userJSON = JSON.stringify(user);
    this.http.post("http://localhost:8080/user/add", userJSON, httpOptions);
  }
}

UserController

package com.sporthub.backend.controller;

import java.util.Optional;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.CrossOrigin;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.ResponseBody;

import com.sporthub.backend.model.Role;
import com.sporthub.backend.model.User;
import com.sporthub.backend.repository.RoleRepository;
import com.sporthub.backend.repository.UserRepository;

@Controller
@CrossOrigin(origins = "http://localhost:4200")
@RequestMapping("/user")
public class UserController {
    @Autowired
    private UserRepository userRepo;
    
    @Autowired
    private RoleRepository roleRepo;
    
    @GetMapping("/all")
    public @ResponseBody Iterable<User> getUsers(){
        return userRepo.findAll();
    }
    
    @PostMapping("/add")
    public @ResponseBody String addUser(@RequestBody String user) {

            System.out.println(user);
            return user;
    }
}

HTML Form

<form [formGroup]="checkoutForm" (ngSubmit)="onSubmit()">
  <div>
    <label for="name">
      Name
    </label>
    <input type="text" id="name" formControlName="name">
  </div>

  <div>
    <label for="lastname">
      lastname
    </label>
    <input type="text" id="lastname" formControlName="lastname">
  </div>
  
  <input type="submit" value="Add new user">
</form>

Users Component

import { Component, OnInit } from '@angular/core';
import { User } from 'src/user';
import { UsersService } from '../users.service';
import { FormBuilder } from '@angular/forms';


@Component({
  selector: 'app-users',
  templateUrl: './users.component.html',
  styleUrls: ['./users.component.css']
})
export class UsersComponent implements OnInit {

  users : User[] = [];

  checkoutForm = this.formBuilder.group({
    name: '',
    lastname: ''
  });

  constructor(
    private usersService: UsersService,
    private formBuilder: FormBuilder
  ) { }

  onSubmit(): void {

    let name = this.checkoutForm.get('name')?.value;
    let lastname = this.checkoutForm.get('lastname')?.value;

    let user: User = {
      "name": name,
      "lastname": lastname
    };

    console.log(user);
    this.usersService.addNewUser(user);
  }

  ngOnInit(): void {
    this.getUtenti();
    console.log(this.users);
  }

  getUsers() : void{
     this.usersService.getUsers().subscribe(users => this.users = users);
  }

}