I'm looking to do something similar - spin up EC2 instances, deploy a script to them, run that script with various parameters, have it all defined in some template somewhere in our repo.

Did you look at EC2 UserData at all when you decided to use Ansible? I asked about this use case on the AWS Slack and someone pointed me at it. I haven't had a look yet but if you've used it as well as Ansible then I'd be curious to know how they compare/how you ended up with Ansible + CF instead.