I dont think that you can design a constitution capable of this, because at the end of the day, a constitution is just words on paper, so if you can get enough people capable of violence to follow you, you can simply directly violate the thing and declare yourself dictator, and an elected executive must by being elected have a significant group that at least somewhat approves of them, and by being an executive have some ability to ensure the law is carried out implying the capacity for violence. You can try to weaken the executive to make this more difficult I suppose, but you probably cant make it impossible without breaking the functioning of the executive completely, and you also need to avoid a case where one of the other two branches seizes control from the rest as well.

Ultimately what you need for a healthy democracy is two things: an election system that actually represents the wishes and interests of the people, which is anywhere from very difficult to not technically truly possible, and a populace that cares enough about their system to not use their electoral power to elect someone (or pass laws in the case of a direct democracy) that demolish or usurp that system. The US fails at both of those at the moment, the latter possibly in part due to a long time deficiency in the former.