Authoritarianism is unavoidable and good
A true elite when voting will make the correct decision more often than any individual human can
x = probability of a senator making a decision
y = probability of the senate making said decision
If you expand the senate too much in size however you will decrease the incentive for individual senators to vote correctly and you will make it harder to maintain high quality among senators (having to accept less qualified people to fill the seats).
Thus pure elite rule is the form of governance with the best potential.
Societal survival of the fittest
Weak societies will fall and be taken over by stronger societies.
Democratic societies tend to become weak and this cannot last forever, you can utilize weaknesses in democratic systems to slowly grab power, after that there will not be any more real democracy.
We are currently seeing the West moving towards authoritarianism to counter Russia and China.
Because elite rule has the highest potential we can expect a country with elite rule to expand to the point where it governs the entire planet (beating all competition). If somehow a democratic country wins this competition it will not last due to internal instability.
Why a ruling elite will take care of their citizens
A farmer will take care of his animals because they are his property and beneficial for him.
Similarly with pure elite rule the elite will naturally want to take care of their citizens since they are the base for the power of the elite.
So we can expect countries with elite rule to aggressively favor their own citizens to expand the base of their power (conquering new areas and growing the size of the population).
They will also make people believe strongly in the government by controlling media and via effective governance.
Once the ruling elite has gained control over the entire planet they will be able to focus more on creating a society that is good for the average citizen since they will no longer have to compete against other governments for territorial control.
Why separation of power is a bad idea
One obvious issue with trying to separate different branches of the government is that then it's not clear where the actual authority is, then you will need to have some body (such as supreme court) to determine where the authority of different bodies begin and end.
In addition having different independent bodies working together will create multiple points of failure. A good example why this is a bad idea is the tetrarchy system implemented by Diocletian, having 4 largely independent emperors was more efficient short-term for defending the empire but it depended on the emperors not turning against it other, it later devolved into civil war.
While you can have specialization within the government it's very important that the highest power has a good knowledge about the society as a whole. You can have different senators specialize at different tasks but then it's important that people make sure the senators entrusted with specific tasks actually do a good job.
About checks and balances to power
The power of an entity can be limited by hard limit or by deterrence.
For a hard limit to work when it comes to government decisions the time it takes for other individuals to intervene cannot be too long, only a few minutes in the case or nuclear war.
deterrences can often be evaded, there are many ways to cheat i elections and in wars you may be able to limit the damage that can be delivered in return if you decide to push the nuclear button.
If someone does a crime it will often be too late once the police show up and many people are willing to break the law to achieve a political goal. Brenton Tarrant killed 51 muslims knowing he would go to jail for it, breivik killed 77 individuals and didn't even expect to survive.
Hard limits to the power of certain leaders only changes the power distribution, it doesn't limit the total power of the government. In order to limit the power of one entity (such as the president) you need to empower some other entity (such as the supreme court).
Let's say you make it harder for the government to convict (such as requirering 75% majority in the courts including the supreme court) in that case you can just illegally kill people and then have 3 of 9 supreme court justices block any accountability, the ones doing the killing doesn't even have to be the official government.
Entity A: makes the correct decision 70% of the time.
Entity B: makes the correct decision 60% of the time.
If entity B rules the country but are accountable to Entity A you want entity A to intervene as much as possible, of course it would be even better if Entity A directly rules the country. The opposite true holds true if Entity A governs, in that case it's better if entity B never intervenes.
Unfortunately the ones with the ultimate power will be time-limited and thus they will end up having to rely on other people making good decisions for them. Thus even if the highest court always make the correct decision we might still end up with a lot of bad results due to the highest court not having time to review all decisions made by lower courts.
Separations of power allows for more specialization but it also adds more points of failure, this will only be stable if all branches of government are accountable to the same entity with ultimate power, this specialization can also be achieved by relying on paries outside the official government.
Which is, of course, exactly why authoritarian societies control the entire world now and have conquered many less authoritarian societies in war.
Wait, no, the exact opposite thing has happened.
Russia has actually expanded their borders despite a rather dysfunctional government.
The world is becoming more authoritarian now largely due to what used to be democracies becoming increasingly authoritarian, not due to territorial changes.
China is likely to expand their borders given the opportunity, it's risky for them now due to the US being strong.
What is your explanation of autocratic Russia's army performing way worse in the Ukraine invasion than anyone expected?
Authoritarianism is not automatically better, it just has larger standard deviation when it comes to the success.
So most authoritarian governments will fail. Many will perform worse than democratic governments.
Because Russia's government is rather dysfunctional their territorial expansion isn't going too well now. This is an example of societal selection against incompetent governance.
What's correct about a decision?
From your perspective it's about how it's affect you, people close to you, your offspring and random citizens in the future (in the case of reincarnation).
The senators will naturally want to create a good future for their children (including the ones who are not able to become senators themselves.
I think you're leaving out a crucial detail. Most authoritarian governments will not just fail but reach lower lows that are distinct to authoritarianism when they fail. The ceiling for success is higher, but the floor for failure is lower to a much greater degree. When you say that authoritarianism is good, what you mean by "good" doesn't even seem to align with any common usage understanding of the word.
While the lows might be lower these societies will generally not last very long.
But one of the worst disasters actually took place due to democracy