I have a legacy WP blog that I wanted to migrate to some static architecture for ages but IMHO users should be able to comment and maybe even post a pingback. I know, old MT days. But social media is always about getting (positive) comments and feedback, not just dropping statements and knowledge.

I also don't want to tie my site to disqus or other 3rd party cloud services and their implication on GDPR.

Have you seen Talkyard Blog Comments? It's open-source and based out of Europe. https://blog-comments.talkyard.io. (I'm developing it.)

> legacy WP blog ... users should be able to ...

(What's your blog about?)

A buddy found Cusdis - a self-hostable Disqus alternative: https://cusdis.com . But this only does comments, no pingbacks - and via a separate product that you need to integrate using JavaScript.

And running this opens you up to security issues you were trying to avoid by going the SSG route. In a way you could just keep WP, then.

(Also, WP has this beautiful ActivityPub plugin that makes your blog a fediverse account that people can subscribe to and even comment on your posts from Mastodon/Pleroma/etc.)