I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to "Accept " the answer.
Issue: How to configure via Bicep the routing intent policy for a secured virtual hub in a vWAN to set the internet security configuration selectively for individual connections as described in the portal under the security configuration section of the Firewall Manager.
Solution: In testing various deployments via the portal vs bicep, op has discovered that in bicep, the property enableInternetSecurity on the vHub connection resource is what determines the selective internet security configuration for individual connections, but it only works if the main routing intent policy is configured to route internet bound traffic through the firewall. So, the routing intent policy acts as a master switch to enable the option for routing internet traffic while the enableInternetSecurity property is what determines which connection will use the routing option for internet bound traffic.
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