If that would be possible, how would you moderate comments, seeing how random things can get?

I don't know what you mean? If I am the admin of an instance or the moderator of a group, I could delete comments or is this just not possible?

Federating with only approved finstances (federated instance)?

Why doing this? Wouldn't it be enough to block the illegal instances and those who are explicitly against your topics?

What if you keep your blog, then push every post you make there to your solo-community on a finstance? You can engineer your comment section on the blog to pint here or fetch the comments content from fediverse to your blog…

I am trying to be as green as possible. Having a blog on one server and the comments on another sounds like an inefficient way of using resources. Why not just put the articles where the comments are?

With Mastodon I had the same idea, that I will publish an article, post a link with short description on Mastodon and then use the Mastodon post as the comment section, then edit the blog article and put the link to Mastodon on the end of the article with a simple text link like "Comment section".

But even this idea felt a bit odd and more unprofessional.

Lemmy looks like a really good solution to this atm.