> Very early in the empire, a law made all conquered people officially Roman
Wouldn't call this "early" in the empire. Caracalla [1] did this in 212AD by granting Roman citizenship to free noncitizens, well after the beginning of the empire in 27BC. W Roman Empire fell in the late 470s so about half of the empire's lifetime had severe legal stratification between citizens, free noncitizens (peregrini) and slaves.