hi!! i realise this is from a little while ago but i just stumbled across it and i also do xerxes for personalities in their times so i thought i’d answer in case you still wanted some insight!
as my school taught it, legacy refers to both material legacy and historical legacy:
material legacy focuses on xerxes’ physical impact on the persian empire — namely persepolis. this also relates strongly to his cultural and social legacy.
historical legacy refers to the enduring impact left behind by xerxes’ reign, and applies to a bunch of different aspects of his rule, including administrative, economic, social, cultural, religious, and so on. so it’s good to discuss his impact (and therefore legacy) as it relates to these areas. for example, for administrative legacy you could talk about how through his organisation of the persian empire through the royal court and satrapy system he left behind a legacy of stability that endured throughout artaxerxes’ rule (source: richard stoneman). or for religious legacy you could discuss his worship of ahuramazda and his treatment of other religions within the empire (sources: the daeva inscription, the writings of josephus) and how that continued to shape the persian empire past his reign.
hope that helps!!! generally speaking pretty much anything in his life counts towards his legacy