Is the reason possibly AGPL Javascript? Maybe then server side code, could be AGPL, and Javascript instead e.g. MIT?
However don't one typically place a blog commenting system in an iframe. Then personally I think you wouldn't have to release the website that embeds the blog comments iframes, under AGPL? And a discussion forum, one places on a sub domain, on a separate server? Also separate from your main website?
Background: I'm developing AGPL discussion software that you can also embed in an iframe, for blog comments. https://www.talkyard.io/blog-comments. What are your thoughts about using sth like that, if the code that creates the iframe is MIT or Apache2, and the code that runs inside the iframe is AGPL?
From a commercial perspective, I think you would be better off with the AGPL as well (perhaps dual licensing). I don't think you'd be able to sell such software on a per-license basis anyway (or, at least you'd not make much money at it as the audience is pretty limited). I think you'd be much better off positioning yourself as an expert in the area and selling contract services for other (perhaps proprietary) systems. Cheaper to hire you than to implement it themselves while tip-toeing around your implementation which they can't infringe.