When William of Orange became William III the settlement instituted was one based on the idea of limited, mixed monarchy. However this was arguably more about the conditions under which he became King than his personal beliefs, as these were the terms under which he was offered the crown, so not sure if it really counts or not for the purposes if your question.

Are you asking for kings/emperors/leaders before 1900s who have resigned without any external pressure? Or did I get it wrong?