Import from GDrive doesn't work

Having now migrated my data from Google to DB, I thought I'd post a follow up of what I actually did.  I hope it helps others as there's a lot of misinformation out there.

1. Firstly, and sadly, there is no (as yet) solution to large scale migration to DB.
2. Given that part of the Google lock-in is the reliance upon their Office tools - slides, docs and sheet.  These will not migrate, however, it is possible to save time in converting by using the Google take out feature - https://support.google.com/accounts/answer/3024190?hl=en. By configuring the output to "Add to Dropbox" Google will do the MS Office conversion for you.
3. Another tip is to limit your Zip files to 10Gb - as tempting as the 50Gb option is - since these can (usually, but not always) be unzipped online.
4. If you're transferring video files that exceed this size, I'm afraid there's no simple solution.  You're going to have to download them from Google and then upload them to Dropbox.
5. If you've got a complicated folder structure like I had, grab a tool like VisualDiffer (by Pablo J. Malacara).  By using this tool I was able to ensure that I accurately reconstituted my files and folder structures when unzipping the take outs.
6. A tip with VisualDiffer: The tool works smoothly when comparing files locally on my Mac, as well as those in iCloud, and even DB.  The issue comes when trying to use VisualDiffer to sync between GDrive and anywhere else.  It will just pop!
7. The £10 / month upgrade to Virgin Media's fibre has been invaluable.  I've spent many hours sat at my kitchen table with a network cable wired into my hub to get the best speed possible, and it really does make short work of large transfers.
8. Although I'd previously uploaded all of my backup data DVDs, I've used this opportunity of having to download my data to take snapshot and build a local replica on SSD.
9. Going forward, I'm maintaining a local copy of my working body of data in DEVONthink, with a snapshot held on SSD, and I backup to DB.  This setup gives me an awesome search and retrieval system, and the security of SSD and cloud backups.

Although it seems sensible to seek a cloud-to-cloud migration tool or service, I was unable to find one that met my needs (i.e. I needed to transfer files >10Mb which is the vendor limit I ran into).  I can understand the commercial need to lock-in data, buyer beware.  

The DB transfer feature didn't work for me at all using Crome, and I had occasional success using Safari, but not frequently enough to inspire my confidence in using it.  As you'll read above, the support position is RTFM, but I found this misleading as it suggests a transfer is possible, but it's actually a download followed by an upload.

The long and short of it is this, you're going to have to download your data.  I made peace with this and just ground my way through it.

I hope others have an easier time than I did.