I asked if they have a secure mechanism for file uploads and they responded "email is fine". No, email is not fine.
They _really_ struggled to understand why I wouldn't just email them every document they asked for (from birth certificates, marriage certificates, 6 months of financial data, 12 months of pay slips, and a host of other things).
In the end I hosted them myself and phoned them with a password, then deleted them once I'd seen they'd been accessed.
Unfortunately, I have no confidence that they have, did nor will handle all of that information appropriately once they receive it.
How is it not complete insanity to require _that much_ information on someone and _not_ have strict training and liability for managing it?
Between then and now, Firefox Send has come and gone, but fortunately it's open source, so I'll host that if I find myself in a similar situation in the future.