MAC - Importing Legacy POP On My Computer data

The end is nigh for Legacy thus I did the switch as a test knowing I could still revert.

Background:

I have long time POP account. In the UBF8T346G9.Office, Outlook, Outlook 15 Profiles, Main Profile: 

• Data folder: 159 GB. Largest folders:
  • Message Attachments: 90 GB
  • Message Sources: 65 GB
  • Messages: 5 GB

Before I switching I tried:

• Export Data to OLM – don't have enough room
• Delete attachments from multiple emails – can only do one email at a time. Rules can't help 

After switch it began importing On My Computer data which was hopeful, but it held at 141498 items, and would take days I read. I reverted back to Legacy and will go another way about it when I have a few off days.

My question:

Apart from deleting years of emails, what's a better way to get the most data out of Legacy, and/or speeding up the import? 

• Get another drive and Export Data to OLM, then manually import?
• Delete attachment folders directly from Message Attachments folder? 
• Find something like Clean my Mac?
• ??

Perfect world I get all my in out mail with whatever attachments remain and Calendar data, but I'll take what I can get at this point.

Appreciated!