Models/User.php
    public function exports(): HasMany
    {
        return $this->hasMany(Export::class);
    }

UsersObserver
    public function deleting(User $user): void
    {
        $user->accessTokens()->delete();
        $user->roles()->detach();
        $user->permissions()->delete();
        $user->exports()->delete(); // добавил
    }

ExportsObserver
    public function deleting(Export $export): void
    {
        Storage::disk('public')->delete($export->filepath);
    }