ImgX can be used to combine multiple images into a single multipage TIFF file.  An easy way to do this and keep memory usage low is to use the AppendTIFF method.  Here is an example:

Private Sub CombineImagesToTiff(ByVal outputFile As String, ByRef inputFiles() As String)
    Dim i As Long
    Dim lb As Long
    Dim ub As Long
    Dim exists As Boolean
    
    Dim ix As ImgX
    Set ix = New ImgX
    
    exists = Dir(outputFile) <> ""
    lb = LBound(inputFiles)
    ub = UBound(inputFiles) - 1
    
    For i = lb To ub
        ix.Import.FromFile inputFiles(i), 0
        If Not exists Then
            ix.Export.ToFile outputFile, ixfsTIF
            exists = True
        Else
            ix.Export.AppendTIFF outputFile
        End If
    Next
    
    Set ix = Nothing
End Sub