Community for newcomers looks like the best option, as long as some regulars curate it. Reducing quality of main forum can essentially destroy it, so it's ruled out.

Discussion section already partially plays this role, as recently there developed a norm that only high-quality posts are acceptable on main site, and low-average-quality posts get actively downvoted, while if moved to discussion, the same posts might be upvoted, since quality expectations there are different. Perhaps the main site can become LW's mathoverflow.net, while discussion site LW's math.stackexchange.com.