It cannot change with 6.0 as SSL inspection is about SSL more than it is about Forti or any other vendor actually.
But between "Deep Inspection" and No Inspection there is a middle step - Certificate Inspection, have you tried it?
The difference is that Forti looks at the certificate SNI values to understand what is the host a user is trying to reach, WITHOUT looking into packet contents. So it does not intercept/proxy SSL connection as Deep Inspection does and accordingly will not cause browsers to display Ceritficate Error message.
And as kind of alternative (kind of as can be easily bypassed by a user) is DNS filtering which blocks not http/https requests to sites but DNS RESOLVING of those sites. It btw requires licensing and as I said can be circumvented by a prepared user.