I agree Terraform isn't great for installing software. Cloud-init works but it can be cumbersome. If that is your major pain point though then I'd look into EKS and using Kubernetes to manage and install software. There is a CLI tool called eksctl that makes setting up an EKS instance a breeze. I don't know all the intricacies with AWS, but with Google Cloud you can setup a single node "cluster".

If you're already planning on standing up at least 3 compute instances though, might as well run EKS in a cluster.