I feel like you are confusing several things at the same time. First of all, it is not like everything ionic is easily soluble and dissociates. Second, just because something is made by reacting metal and non-metal doesn't necessarily mean the compound is ionic, much more important thing is the electronegativity difference. Third, while some oxides are definitely ionic, they never dissociate, at best they react with water. Finally, heavy metal oxides are quite often quite inert.