Besides, it's been "tried" already, in this form: https://en.wikipedia.org/wiki/Separation_of_powers - and by now it's clear how much that helped, right?
In reality, "the separation of powers" was just a bullshit PR-distraction, meant to give us a false sense of security, to alleviate the nagging subconscious awareness that the idea that we live in a system that benefits us just makes no sense whatsoever.
Regardless of how parts of the government are ostensibly "separated" or "firewalled", they're still all part of the same overall organization: the government.
There's no way to arrange an all-encompassing "authority of coercion" in a way that benefits the people being coerced. Another way to look at it is that forcefully taking people's money does not benefit whoever's money is taken. It benefits the takers.
The exploiters need the masses of the exploited to be blind to the exploitation, because otherwise it would come to an end, and that's why we find the idea of "the separation of powers" floating around, along with "the social contract" and "consent of the governed" and so on.
Oh kind of like The Constitution, for example? Obviously that hasn't worked out too well in the long term, what with the US being a burgeoning police state and all.
But if "we" write a new and more sternly worded piece of paper that says "rulers can't mistreat their subjects", that should work, right?