As much as any of the cities of Slavers’ Bay are not actually Valryian, would Tolos, Elyria, and Mantarys be considered ‘Ghiscari’?

No. Tolos, Elyria, and Mantarys were colonies founded by Valyria.

 New Ghis, Ghaen, Yaros, those are arguably “Ghiscari,” although that gets into a larger discussion about invented nationalisms.