I'll be honest, I'm not a big fan of Sphen.
1. you describe it as being in an sea, while we've pretty much established that the largest body of water connected to the city is the Lake. Boats from the Port District and the Lake District only reach seas and oceans through Gates.
2. the basic idea isn't bad, but it's simply not unique enough. As it is, it may as well be an expanded are of another district linked via Gate.
3. The way it's described in your last posts, makes it sound like the island competes with Ishka:

"Also, it's rumored that deep in the islands bay, there's something, and it stops all the tension in Ishka from becoming a full-fledged war."

This isn't practical with a city the size of Ishka.