We've been tasked with printing a 70,000 record A4 mail merge.
The idea of creating a 70,000 page document in Data Merge fills my iMac and I with fear, so is there a way to automate this directly when printing, or are there other solutions?
Yep. At a certain scale it no longer makes sense to do it oneself. Talk to your printing company about their VDP (variable data processing) capabilities. It will not only be sorted, but route sorted. They can handle the postage (which is less expensive once route sorted), etc. At least if this all gets mailed...
Otherwise, break it into chunks and go to PDF, print PDFs.