What is the best way to set up a CI/CD pipeline on ECS?
The question is a bit broad, but here are some suggestions to accomplish what I believe you are asking. If they are simple or not is for you to judge!
If the infrastructure referred to already exists and you would like to manage, update using infrastructure as code (IaC), you could try tools to convert the infrastructure to code. (These are new and at least one is in beta at the time or writing this)

Existing Infrastructure to Terraform:
https://github.com/GoogleCloudPlatform/terraformer
Existing
Infrastructure to CloudFormation:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/cfn-using-cloudformer.html

If the infrastructure does not exist, I recommend using infrastructure as code (IaC). AWS was mentioned, as well as the complexity of CloudFormation, with these considerations, Terraform could be a good option.

Cloud Posse offers some good modules to simplify using Terraform to build infrastructure. This module may be a good place to start.

Finally, if you would like to abstract away the bulk of the infrastructure and just run containers in the cloud.

Google Cloud Platform (GCP) offers Cloud Run, providing a completely managed service to run containers in the cloud with out much of the underlying complexity. The GCP Provider for Terraform can help here.
AWS Fargate cloud be another option, with Cloud Posse again offering modules for Terraform.

Once the code and infrastructure option is selected, CI/CD is as easy as adding a buildspec.yaml to the repository and adding the Code Build infrastructure to the Terraform code (AWS) and similar for Cloud Build (GCP).
Hopefully these options help find you the simple solution you are looking for!