The only point I disagree with is Terraform. It is brilliant for this exact scenario because it's self documenting. When you do need to update those SPF records in two years time, having it committed as a Terraform file is much better than going through (potentially stale) markdown files. It's zero maintenance and really simple. Plus its ability to weave together different services (like configuring Fastly and Route53 from the same place) is handy, too.