To me, a sane, educated, and healthy society with a legitimately working legal and justice system should never fall to fascism.

That's not really how it works. Fascism doesn't arise out of a lack of those things, it arises because of the capitalist drive for endless growth encountering the tendency of the rate of profit to fall as economies become more developed. When there isn't a lot of room to grow, fascism presents a solution in the form of artificial growth through cannibalizing minority businesses. As this cannibalism continues, the definition of the in-group continuously narrows to ensure there's always a target.

No matter how many checks and balances there are or how well functioning the system is, the material pressures will eventually overwhelm it, and there will either be nationalizations to remove the profit motive from already developed industries and remove the pressure, or there will be enshittification, economic cannibalism, and fascism. "Socialism or barbarism."

Systems only matter to the extent that people with guns say they do. We have a check on the sorts of things Trump is doing, SCOTUS ruled that he has to return a prisoner sent to El Salvador, and he's just opted to ignore it. And it turns out he can just do that even though it's illegal because SCOTUS doesn't have guns and the people who do have guns don't care.