Update Choice Field using CSOM in PowerShell

Param(
 		[Parameter(Mandatory=$true, position=0, ValueFromPipeLineByPropertyName=$true, HelpMessage="Specifies the Web. The type must be a valid URL or GUID")]
        [String]$WebUrl
	  )
Set-Location $PSScriptRoot
Add-Type -Path ( ".\SharePoint Assemblies\Microsoft.SharePoint.Client.dll")
Add-Type -Path ( ".\SharePoint Assemblies\Microsoft.SharePoint.Client.Runtime.dll")

# help to use lamda expressions to query object properties
. .\Load-CSOMProperties.ps1

function Add-ChoiceValueToField {
  [CmdletBinding()]
  param
  (
    [Parameter(Mandatory=$true,Position=0)]
    [Microsoft.SharePoint.Client.ClientContext]$ctx,
 
    [Parameter(Mandatory=$false, position=1, ValueFromPipeLine=$true, ValueFromPipeLineByPropertyName=$true, HelpMessage="TODO")]
    [Alias("ListName")]
    [string]$ListTitle,

    [Parameter(Mandatory=$false, position=2,ValueFromPipeLineByPropertyName=$true, HelpMessage="TODO")]
    [string]$FieldName,
    
    [Parameter(Mandatory=$false, position=3,ValueFromPipeLineByPropertyName=$true, HelpMessage="TODO")]
    [Alias("Field")]
    [string]$ChoiceValue
  )
   #Retrieve List
    $List = $ctx.Web.Lists.GetByTitle($ListTitle)
    $ctx.Load($List)
    $ctx.ExecuteQuery()

    #Retrieve field
    $field = $List.Fields.GetByInternalNameOrTitle($FieldName);
    
    $fieldChoice  =   [Microsoft.SharePoint.Client.ClientContext].GetMethod("CastTo").MakeGenericMethod([Microsoft.SharePoint.Client.FieldChoice]).Invoke($ctx,$field)
    Load-CSOMProperties -object  $fieldChoice -propertyNames @("Choices") ;
    $ctx.Load( $fieldChoice);
    $ctx.ExecuteQuery() ;
   
  #if not exists then add 
    $bExists = $false
    $FieldChoiceValues=@(); 
     foreach ($choice in $fieldChoice.Choices)
      {
       $FieldChoiceValues +=$choice
       if($choice -eq $ChoiceValue)
       {
         $bExists = $true
       }
     }
                    
#add choices to dropdown list
  if($bExists -eq  $false)
       {
         $FieldChoiceValues +=  $ChoiceValue

         $fieldChoice.Choices = $FieldChoiceValues;
         $fieldChoice.Update()
         $ctx.ExecuteQuery() ;
       }
  }
    $ctx  = New-Object Microsoft.SharePoint.Client.ClientContext($WebUrl) 

   # $ctx.Credentials = System.Net.CredentialCache.DefaultCredentials;
    $web = $ctx.Web

    $ctx.Load($web)
    $ctx.ExecuteQuery()

 Add-ChoiceValueToField -ctx $ctx -ListTitle "ListTestManually" -FieldName "TypeNum" -ChoiceValue "7"