I'm not a believer in absolute hereditary Catholic monarchy either ... for the same reasons that St. Thomas wrote that, yes, it might be ideal in some respects (not sure about the hereditary part even then), but it's fraught with problems due to our not living in an ideal world due to Original Sin. We had scuм like Henry VIII take an entire country into schism and heresy, just because he could. We've had many tyrants cause people to hate the Church and rebel. We've had many incompetents that have ruined their countries. We've even had insane mentally imbalanced types, sometimes due to inbreeding ... just because they had the right "blood line".
Here's how I would lay out an ideal form of government ...
Monarchy ...
Constitutional Limits on Power ...
... including Veto Power given to Church (bishops of the country, and state could appeal to Pope)
Then there should be representation of the People to prevent the elite from becoming tyrannical (even when they haven't crossed the line into measures that would be vetoed by the Church)
In addition, there should be some kind of a representative body that people would vote for, similar to an aristocracy, and these would exert veto power over a Monarch that's more aligned with what's good for the people rather than moral questions that the Church might veto. So this group too would have veto power.
Prospective Candidates would be required to pass certain intellectual qualifications, and also take an oath of loyalty to the Church in addition to loyalto tot he State.
Each Candidate would be permitted to issue a video or two, with equal time alotted to candidates, on a single centrally-hosted website, where they can explain themselves in summary, and then they can list some of their positions on the website. No private campaigning will be permitted that would allow people to buy their way into office. Candidates must have the endorsement of their Catholic pastor, and can be vetoed or rejected by the bishops.
Prospective Voters would be required to pass certain tests, including some quizzes about the positions of the different candidates, before being permitted to vote ... this to prevent the vote of some informed and thoughtful individual to be canceled out by some moron who votes due to name recognition.
Only men will vote, but a man who's the head of a household willl have as many votes as are in his household, since a man representing a wife and 10 kids should have more weight than a single guy. That's one of the many crimes of the current system. Minors can't vote, but some old jackass in an apartment can vote to go to war and his vote counts as much as that of a father with 10 children, 4 of whom might be drafted to go fight said war. Voters must also swear loyalty to the Church (in addition to the state) and be certified by their pastor as being orthodox practicing Catholics.
Then once the Aristocrats are seated by this voting, they would elect the "Monarch" or Autocrat who runs the country from among themselves. Again, this selection can be vetoed by the Church. In addition, once seated, the Monarch can also be removed by the Church (the bishops) or by the aristocratic body.