using CDK with existing Stack Templates

I have been trying to move towards using CDK along with our current 10 existing stack templates. This would be great since I can load the template using cfninclude and then add any new resources or modify existing resources in the template using code instead of manually doing it in all the templates.
I got a requirement today to add an Application Load Balancer Listener rule to all templates, so when I reached out to aws support on how to access the load balancer in the templates and add the rule programmatically I was told:

Now, to add a listener rule to the listener defined in the template, we will have to create a new
CfnListenerRule resource,

new elbv2.CfnListenerRule(this, "ListenerRule", {
  listenerArn: listner.attrListenerArn,
  priority: 123,
  conditions: [
    { field: "path-pattern", values: ['/'] }
  ],
  actions: [ { type: "forward", targetGroupArn: <target group ARN> , } ]
});

I tried to replicate the same at my end by setting targetGroupArn: tg.attrTargetGroupArn , but unfortunately I could see the error “Property 'attrTargetGroupArn' does not exist on type 'CfnTargetGroup'.”

Therefore, I would suggest you to go with either of these options,

• create the stack first and then hardcode the ARN of the target group.
• Create the listener rule as a resource in the template and then use cfn-include.

These proposed options seem absurd to me, I don't want to hardcode ARN's in my code for every deployed stack and I don't want to keep manually updating 10 different templates whenever I need to make a change, this defeats the purpose of CDK doing the work for me.

I'm seriously questioning the value of using a CDK once there are existing stack templates deployed and in production. Has anyone successfully done this without just creating more over head?

We have decided to just migrate the templates to code, but for anyone who needs to know how to do this, here you go from AWS support:

const template = new cfninc.CfnInclude(this, 'Template', { 
  templateFile: '/Users/abhithm/Downloads/importks.yaml',
});

const lb = template.getResource('LoadBalancer');

const listner = template.getResource('Listener') as elbv2.CfnListener;

const tg = template.getResource('TargetGroup1') as elbv2.CfnTargetGroup;;

const x = tg.ref;

new elbv2.CfnListenerRule(this, "ListenerRule", {
  // listenerArn: listner.getAtt('Arn').toString(),
  listenerArn: listner.attrListenerArn,
  priority: 123,
  conditions: [
    { field: "path-pattern", values: ['/'] }
  ],
  actions: [ { type: "forward", targetGroupArn: x  , } ]
});