If you’re turning down meaningful amounts of money because you consider this information too sensitive to share, you are a crazy person.

I imagine they're willing to accept them a different way. That email doesn't even specifically ask for the forms to be emailed. It just says to "send them over".

Also, I'd argue there are ways to make it reasonably secure over email. An encrypted attachment with a securely pre-shared key doesn't seem too risky IMO.