Is saying: "My Lord, if there is a place for me in Your Jannah, then take me to You, and if it is otherwise, then guide me, then guide me, then guide me, and purify my heart and take me to You" considered ill-mannered towards Allah or a lack of good presumption of Him?

This supplication is not considered a sign of ill-will towards Allah. However, a Muslim should not pray for death unless they fear tribulation (fitnah). The Prophet, peace be upon him, forbade wishing for death due to an affliction in this world, and he commanded anyone who wished for it to say: "O Allah, keep me alive as long as life is better for me, and cause me to die if death is better for me." Wishing for death is permissible when fearing tribulation in one's religion, as the Prophet, peace be upon him, prayed: "And if You intend a trial for a people, then cause me to die without being afflicted." This is also as Umar ibn al-Khattab, may Allah be pleased with him, supplicated.