eks-

eks- strips the title and leaves the person standing. Edzo gives eksedzo, reĝo gives eksreĝo, prezidanto gives eksprezidanto, membro gives eksmembro. The root keeps its ending, and the prefix works as a verb root in its own right: eksigi is to dismiss somebody, eksiĝi is to resign.

It needs a role that can be held and then lost. Ekspatro is nonsense, because nobody stops being a father, and native-feeling Esperanto simply refuses the combination.

For former in the weaker sense of earlier, reach for antaŭa or iama instead. And note that eksedziĝi is the act of divorcing, not a vague memory of having once been married.