I'd go one step further and would generate structured files with the comments, which are stored in an HTTP-enabled S3-bucket, so the website showing the comments can still rely solely on static resources for comment display and only requires AWS Lambda/DynamoDB for posting new ones.
Great idea. Don't forget that Dynamo has a generous free allowance. I'm not sure how much you'd save. Maybe instead of S3 you could use a free CDN such as Cloudflare.
