The less diverse a society is the harder it is to conquer

Or rather, the longer it takes to conquer.

You must first break such a society up into groups with opposing ambitions and needs. If these do not readily exist they are easy enough to create. It just takes time. You must make inroads into the education and media turning the most promising members of the homogeneous culture to your agenda. Entice them with money, sex, power, ego flattery. Once you have control of top level people in education and media you can push your divide and conquer agenda from the top down. Over time, as the agenda becomes rooted, it will push itself from the ground up by the young and impressionable. Those in between will eventually die out.

If the target culture is racially more less homogeneous, then create and exploit gender, sexuality, socioeconomic, religious and political factions. Chip away at the racial homogeneity by increasing immigration of as many diverse groups as possible.

Once you have created oppositional forces within a society you can do essentially anything your heart desires. If one group opposes the direction you wish the target society to go, then you have many other groups whose forces you can marshal to deal with them.

And let's remember, the goal of conquering a people is to make them your willing servants. It is easiest to subjugate those who are unaware of their subjugation.

Over generations the enslavement can become more overt and cruel. Those born into it will think it natural and will thus not think to revolt. Alas, revolt will not be in their vernacular. No will be freedom or the love or longing for it.