Must I repeat the prayers I performed for a year and a half facing the wrong qiblah based on personal ijtihad, and is this ijtihad sufficient to exempt me from repeating them, knowing that I did not imitate or ask anyone, but it was possible for me to ask someone who prays?

If a worshipper at home makes a mistake in determining the Qibla (definition), his prayer is invalid and he must repeat it, because he could have asked or found the Qibla using various means such as mihrabs or by asking a Muslim who could inform him of the correct direction. However, if the deviation is slight, it does not matter.