I didn’t see how mixed-content looks, but from your description it should do the trick.
I think it should be the default setting, and I wonder why would anyone ever want to turn it off? Shouldn’t a post look exactly as its author wrote it? Does it have any other side effect?
I’ve opened this bug as a member in a few forums, not as a forum owner.
It would be awkward if I would have to ask all those forum admins to change a setting that would fix the experience for users like me. I think they expect the forum to look good for all of their users out of the box.