Must Witr be the last prayer before sleep, or is it better to delay it until after the Tahajjud prayer? And in this case, should I pray Witr with the congregation with the intention of it being a supererogatory prayer for them?

It is recommended for the Witr prayer to be the last prayer a Muslim performs at night, based on the Prophet's (peace and blessings be upon him) saying: "Make the last of your prayers at night Witr." The Prophet (peace and blessings be upon him) prayed two rak'ahs after Witr to show the permissibility of praying after it, not as a regular practice.

It is permissible for someone who intends to perform Tahajjud to pray Witr with the congregation and then pray as many even-numbered rak'ahs as they wish, without repeating Witr, or to delay Witr to be their last prayer. If a Muslim performs Witr at the beginning of the night and then wakes up at its end, they should pray what is easy for them in even-numbered rak'ahs without Witr, based on the Prophet's (peace and blessings be upon him) saying: "There cannot be two Witrs in one night." It is not valid to intend a supererogatory prayer with a single rak'ah, because the night prayer is two by two, and no supererogatory prayer consists of less than two rak'ahs except for Witr.