I think it is more like owning a slave. The monarch does not necessarily have to treat his citizenry well because he has control over them.

I sense this is a sentiment that can be brought up against any totalitarian state and don't see any reason to single out monarchism. It is true that many monarchs may feel a sense of masterhood over their country (can't say I really blame them), however finding a monarch who actively mistreats his people is more rare as this negatively impacts their own political power, the stability of their nation, the loyalty of their people, etc. As long as he harbors his feelings in private, it's a non-issue.

All that the monarch needs is to keep the populace working and complacent.

I do not see anything wrong with this because it is an incentive to treat people well, not poorly. When people are made to work through brute force, they are generally very unproductive. (example: North Korea). So the incentive to have a nation of productive workers paying taxes will be filled by allowing a relatively free market in which people may decide their own line of work.

And? Again, this is not inherent to monarchy. If a monarch ascended to power in the United States, he would most likely believe the same "diversity" trash most Americans believe.

The united states is a tricky issue and it is unlikely for a monarchical society to ever evolve there. But hypothetically, I do find your conclusion unlikely because were a monarch to support diversity he would be supporting his own decline. Again, the better alternative for monarchs is to raise the rate of birth within their own population and monarchs are very well aware of this. It's true that for us the people there isn't a guarantee that all monarchs will believe this, and obviously some won't, but it's fair to say that most monarchs will act according to this line of thought.

Immigrants would likely be grateful to the monarch (excluding people like Muslims). Also, you're assuming that monarchs would necessarily agree with your view on immigration, which they would not because there is no ideology inherent to monarchism.

While I agree that there is no ideology inherent to monarchism, I do think that monarchs securing their political power is inherent. This usually means avoiding things that will make them controversial or unpopular, such as splitting their nation up into different cultures through diversity & immigration. Monarchs have a far greater incentive to keep their nations mono-cultural than democratically elected leaders.