Is it permissible for someone who recites what is easy for them from the Qur’an during the day to complete their recitation in the Tarawih prayer, and would this be considered a complete ختمة (Qur’an recitation)?

There is no objection to reciting the Qur’an with the intention of completing a full recitation (khatmah) during prayer and outside of it, and you will be credited with a khatmah for doing so. However, it is preferable to dedicate one khatmah solely for recitation and another for Tarawih prayers, as it is recommended to complete the Qur’an during them. Reciting the Qur’an during prayer is better than reciting it outside of prayer. If you are able to complete two khatmahs, that is superior. Otherwise, there is no harm in combining recitation between prayer and outside of it to achieve a complete khatmah in Ramadan, and it would be most preferable if it were done during Tarawih prayers.