get all choice values from the field using powershell CSOM

You can use the PowerShell command below:

Add-PSSnapin Microsoft.SharePoint.PowerShell -ErrorAction SilentlyContinue

$WebURL="http://intranet.crescent.com/"
$ListName ="Projects"
$MultiChoiceFieldName="Category"

$Web = Get-SPWeb $WebURL
$List = $Web.lists.TryGetList($ListName)
#Get the 1st item stored in the list
$Item = $List.items[0]

#Get the multiple choice field 
$MultiChoiceValues = New-Object Microsoft.SharePoint.SPFieldMultiChoiceValue($Item[$MultiChoiceFieldName])

#Print Each Value
For($I=0; $I -lt $MultiChoiceValues.count; $I++)
{            
 write-host $MultiChoiceValues[$I]
}

Reference: Get-Set Choice Field Value in SharePoint using PowerShell