Laravel エラー/could not be converted to stringについて助けていただきたいです

Object of class Illuminate\Database\Eloquent\Relations\BelongsToMany could not be converted to string

<?php

namespace App\Http\Controllers;

use Illuminate\Http\Request;
use Illuminate\Support\Facades\Validator;
use Illuminate\Validation\Rule;
use App\Models\User;
use App\Models\Models\Tweet;
use App\Models\Models\Follower;

class UsersController extends Controller
{
    /**
     * Display a listing of the resource.
     *
     * @return \Illuminate\Http\Response
     */
     public function index(User $user)
     {
         $all_users = $user->getAllUsers(auth()->user()->id);

         return view('users.index', [
             'all_users'  => $all_users
         ]);
     }

     // フォロー
     public function follow(User $user)
     {
         $follower = auth()->user();
         // フォローしているか
         $is_following = $follower->isFollowing($user->id);
         if(!$is_following) {
             // フォローしていなければフォローする
             $follower->follow($user->id);
             return back();
         }
      }

     // フォロー解除
     public function unfollow(User $user)
     {
         $follower = auth()->user();
         // フォローしているか
         $is_following = $follower->isFollowing($user->id);
         if($is_following) {
             // フォローしていればフォローを解除する
             $follower->unfollow($user->id);
             return back();
         }
     }

    /**
     * Show the form for creating a new resource.
     *
     * @return \Illuminate\Http\Response
     */
    public function create()
    {
        //
    }

    /**
     * Store a newly created resource in storage.
     *
     * @param  \Illuminate\Http\Request  $request
     * @return \Illuminate\Http\Response
     */
    public function store(Request $request)
    {
        //
    }

    /**
     * Display the specified resource.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function show($id)
    {
        //
    }

    /**
     * Show the form for editing the specified resource.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function edit($id)
    {
        //
    }

    /**
     * Update the specified resource in storage.
     *
     * @param  \Illuminate\Http\Request  $request
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function update(Request $request, $id)
    {
        //
    }

    /**
     * Remove the specified resource from storage.
     *
     * @param  int  $id
     * @return \Illuminate\Http\Response
     */
    public function destroy($id)
    {
        //
    }
}

<?php

namespace App\Models;

// use Illuminate\Contracts\Auth\MustVerifyEmail;
use Illuminate\Database\Eloquent\Factories\HasFactory;
use Illuminate\Foundation\Auth\User as Authenticatable;
use Illuminate\Notifications\Notifiable;
use Laravel\Sanctum\HasApiTokens;

class User extends Authenticatable
{
  use Notifiable;

  /**
   * The attributes that are mass assignable.
   *
   * @var array
   */
  protected $fillable = [
      'screen_name',
      'name',
      'profile_image',
      'email',
      'password'
  ];
    /**
     * The attributes that should be hidden for serialization.
     *
     * @var array<int, string>
     */
    protected $hidden = [
        'password',
        'remember_token',
    ];

    /**
     * The attributes that should be cast.
     *
     * @var array<string, string>
     */
    protected $casts = [
        'email_verified_at' => 'datetime',
    ];

    public function followers()
   {
       return $this->belongsToMany(self::class, 'followers', 'followed_id', 'following_id');
   }

   public function follows()
   {
       return $this->belongsToMany(self::class, 'followers', 'following_id', 'followed_id');
   }

   public function getAllUsers(Int $user_id)
   {
    return $this->Where('id', '<>', $user_id)->paginate(5);
   }

   // フォローする
   public function follow(Int $user_id)
   {
       return $this->follows()->attach($user_id);
   }

   // フォロー解除する
   public function unfollow(Int $user_id)
   {
       return $this->follows()->detach($user_id);
   }

   // フォローしているか
   public function isFollowing(Int $user_id)
   {
      return $this->follows()->where('followed_id', $this->follows())->where('followed_id', $user_id)->exists();
   }

   // フォローされているか
  public function isFollowed(Int $user_id)
   {
       return (boolean) $this->followers()->where('following_id', $user_id)->first(['id']);
   }

}

@extends('layouts.app')

@section('content')
    <div class="container">
        <div class="row justify-content-center">
            <div class="col-md-8">
                @foreach ($all_users as $user)
                    <div class="card">
                        <div class="card-haeder p-3 w-100 d-flex">
                            <img src="{{ $user->profile_image }}" class="rounded-circle" width="50" height="50">
                            <div class="ml-2 d-flex flex-column">
                                <p class="mb-0">{{ $user->name }}</p>
                                <a href="{{ url('users/' .$user->id) }}" class="text-secondary">{{ $user->screen_name }}</a>
                            </div>
                            @if (auth()->user()->isFollowed($user->id))
                                <div class="px-2">
                                    <span class="px-1 bg-secondary text-light">フォローされています</span>
                                </div>
                            @endif
                            <div class="d-flex justify-content-end flex-grow-1">
                                @if (auth()->user()->isFollowing($user->id))
                                    <form action="{{ route('unfollow', ['id' => $user->id]) }}" method="POST">
                                        {{ csrf_field() }}
                                        {{ method_field('DELETE') }}

                                        <button type="submit" class="btn btn-danger">フォロー解除</button>
                                    </form>
                                @else
                                    <form action="{{ route('follow', ['id' => $user->id]) }}" method="POST">
                                        {{ csrf_field() }}

                                        <button type="submit" class="btn btn-primary">フォローする</button>
                                    </form>
                                @endif
                            </div>
                        </div>
                    </div>
                @endforeach
            </div>
        </div>
        <div class="my-4 d-flex justify-content-center">
            {{ $all_users->links() }}
        </div>
    </div>
@endsection

// ログイン状態
Route::group(['middleware' => 'auth'], function() {

    // ユーザ関連
    Route::resource('users', 'UsersController', ['only' => ['index', 'show', 'edit', 'update']]);

    // フォロー/フォロー解除を追加
    Route::post('users/{user}/follow', 'UsersController@follow')->name('follow');
    Route::delete('users/{user}/unfollow', 'UsersController@unfollow')->name('unfollow');

});