First, check out the community version of this module in our DSC Hub (in the Git repo). I know it accepts a thumbprint for a certificate, for example. And I think you have to create multiple configuration items for each binding. The “c” versions are usually less buggy and more-featured than the “x” versions; the guys have been enhancing the “x” versions as they use them in production.
There’s no way to pass in an SslFlags value with Microsoft’s original “x” version of the resource, but we have a cWebAdministration version up on the PowerShell.org GitHub site, and you could add it to that. (Should be pretty straightforward to add an extra property to the DSC resource and pass it along to the underlying IIS commands.)