It depends. There should be a way to find and index those user comments, if they have a positive SEO value. This conflicts with the goal, that blog post itself

It depends. There should be a way to find and index those user comments, if they have a positive SEO value. This conflicts with the goal, that blog post itself should be at delivered as a static content, at best from a static file, to withstand a HNDDoS even on low budget. So one solution could be to run the comment system as AJAX, moderate user comments a few times a day. Click [x] on every comment, you want to appear in the static page, and [generate comments] to generate a new static page, with the new public available comments. Give each user a cookie and a localStorage, to show their comments in their own AJAX version, even if you blocked them from both the AJAX comment system and the public one.