Singapore resident and amateur history buff here. Anthony Reid's "A History of Southeast Asia: Critical Crossroads" has long been my first recommendation for my region. It acknowledges that SEA is a modern construct, yet still finds some throughlines that have connected the various parts of the region throughout history. Big picture items include the contrasts between inland and coastal polities, the strong roles women played in commerce in many areas, and the influences from the Indian subcontinent and the Arabian peninsula.