I have tried to do the same thing – bulk upload.
I have exported a CSV file from one website, and tried to import same file on a new website – it locks up at 1% with server 500 error:
Was crashing after a PHP error “Uncaught Error: Cannot use string offset as an array” on line 340 of the file “wp-content/plugins/download-manager/src/Admin/Menu/ImportExport.php”.
I have checked your sample CSV and many of the generated/exported header names don’t match – why is that? shouldn’t your plugin generate same header names in the CSV file??
eg:
Your sample: title
My CSV: post_title
Yours: no ID (no ID column in your sample)
My CSV: ID
Yours: description
My CSV: could not find (no column exported)
Yours: url_slug
My CSV: post_name
etc.