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Mastodon is a great option. It supports text, pictures, and videos and is focused on small replies. Perfect for a comments section on a blog post. (It's also very popular, so your users can easily find additional help online.)
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SaaSHub
SaaSHub - Software Alternatives and Reviews. SaaSHub helps you find the best software and product alternatives
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Friendica is powerful, but it's also very complex. If your primary goal for the server is simple discussions about your website, Friendica might be more than you need.
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WriteFreely
A clean, Markdown-based publishing platform made for writers. Write together and build a community.
If you're looking to publish long form content, you should also look at Write Freely. It's a blogging platform where the fediverse is baked in from the start. The plug-in you found will work great, especially when paired with a Mastodon instance. But it might be worth a look if you haven't seen it already. (That said it sounds like you want to deeply structure your content, so Write Freely might not be enough for your goals anyways.)