I get that riot wants to support a variety of builds and that's really cool on the champions it works on.
Ekko is not one of them. Tank ekko just removes most of his actual weaknesses and isn't remotely fun to play against. Riot's tried to nerf it in a way that keeps it viable, and the result is that it's still his best build and it's still stupid to play against.
At some point you have to just prioritize fixing the problem over leaving the offbuild viable. Just bite the bullet and make it so that tank ekko isn't even remotely viable, and then buff his AP playstyle if it's too weak.