The reproductive advantages do not scale with wealth and power in general.

You seem to be confusing monarchs in general with a particular instance of a conquerer that was not born of a monarch, the a founder of a monarchy or in any way a king or prince.

Rulers who used their power and wealth to acquire enormous harems and then reproduced like crazy, leaving hundreds or even thousands of children, are not at all uncommon historically. Furthermore, the children of regular royal concubines were typically not in danger of starvation, and thus had a decent chance of reproducing themselves, while the most favored sons would normally become powerful enough to amass their own harems with time. This seems like pretty good scaling.

The Christian idea that the ruler is bound by the same moral standards of monogamy as his ordinary subjects is a huge outlier among human cultures. (In fact, I can't even think of any other similar historical example, though someone more knowledgeable could probably find it.) Certainly, if you look at almost any other place and time, you'll find rulers reproducing at rates unthinkable even to their high-ranking subjects.