Is it better to leave with the first Imam after eight rak'ahs and witr at home, or to complete the prayer with the second Imam (shaf' and witr) and then perform tahajjud without a second witr, in order to gain the reward of Qiyam al-Layl?

If you intend to pray after sleeping, it is better to delay Witr so it is the last of your prayers, due to the Prophet, peace and blessings be upon him, saying: "Make the last of your prayers at night Witr." It is possible to combine the virtue of praying with the Imam and delaying Witr by having the congregant add an extra rak'ah to the Witr that the Imam prayed, then performing Witr after his voluntary night prayers.