How to insert the document to the end of the Word doc instead of the beginning?
Use the end of content property the get the end position of the text object.

Here's the script, tested on Microsoft Word Version 16.13

set FilePath to choose file
my AddAttachmentFileToWordDoc(FilePath)

on AddAttachmentFileToWordDoc(FilePath)
    set f to FilePath as text
    tell application "Microsoft Word"
        set StartRange to (end of content of text object of active document) - 1
        set theRange to create range (active document) start StartRange end StartRange
        insert file at after theRange file name f with confirm conversions
    end tell
end AddAttachmentFileToWordDoc




An alternative to a range, you can use after last character of active document, like this

on AddAttachmentFileToWordDoc(FilePath)
    set f to FilePath as text
    tell application "Microsoft Word"
        insert file at (after last character of active document) file name f with confirm conversions
    end tell
end AddAttachmentFileToWordDoc