This was already examined once before in More options when flagging for migration, but in the past year the network has grown a lot.
Questions about pure math, electronics, physics, and other tangentially-programming-related topics are relatively uncommon on StackOverflow, but they certainly come up and those sites are mature enough to serve askers better than SO.
Why not allow power-users who know the scope of SE to manually type the name or URL of an SE site below the five blessed options? It feels bad to label something as "offtopic" when it's really sorta relevant and more out-of-place.
Asking for moderator attention is something many users just hesitate to do on a good-faith OP. Getting five votes is part of the process. Would-be flaggers might be more comfortable with a system where a vote-to-close results in the question being sent to a moderator queue. The mod might make sure the selected site is suitably mature and active to receive the question.