Don’t things like Freenet do similar?

Except that every user is also a node, thereby mixing their personal traffic into a share of network traffic. Or so I understand it.

I'm not sure. Freenet actually stores information, this is pure communication system. I don't think it uses dummy messages.

My target size is also <500 lines, and I think <200 is feasible, whereas Freenet is apparently 192,000 lines.