You’re not the first to start thinking of a comment system. When searching this forum, the following solutions based on Javascript pop up:
But that doesn’t exactly make the comments static.
I don’t like the idea of CSVs. The CSV format is quite fragile wrt. escape sequences.
The best idea I’ve heard for static comments, that I liked, is to use the GitHub API to create comments based on creating web-based pull requests. Then have a CI check that validates that it’s a valid comment (i.e. does not delete anything and only appends in allowed places). This approach solves authentication (you need a GitHub account to comment), and it solves where to host the submit form and the server-side validation, and it integrates neatly with re-publishing the site; and you can decide whether to auto-merge if CI approves that it’s a legal comment, or moderate every comment, or something in-between.