Eh, any way you do it could leave it in an unfinished state if interrupted, I'm not too bothered about that. (But it does sound like you think I was speaking in favour of doing it in a procedural python script sort of way? I was not.)

Packer and Terraform do different jobs (they're both by Hashicorp!) - you can bake an immutable image all you like, you still need to get a server, put the image on it, give it that S3 bucket it needs, IAM, etc.