Yeah, I agree with you in principle. I merely think we need to exercise caution in how to implement it in practice.

If our generations ever get the opportunity to do major reforms or even overhauls of government institutions, organizational structures, foundational principles, and key functions, we need to remember not to throw the baby out with the bath water for one thing.

For another, we need to avoid tunnel vision. It's so easy to get caught up in the needs of the current political moment, and lose sight of the bigger picture. That's why I would caution against changing constitutions for mere political expediency, or as a means toward temporary goals.

It should only be done to update the structures and functions of government to be better suited for ensuring enduring principles such as liberty, justice, and human rights (as long as those terms are clearly defined so they can't be distorted or corrupted in the future; I'm talking Rawlsian and Senian views of justice, and liberty should be about protecting people from oppression, not affirming the entitlement of the powerful to anything-goes. So maybe a more Keynesian view of economics).

As for your ideas of an immutable core, I generally agree. We might need to expand on it further, and I think that task is largely suited to career academics with multidisciplinary expertise ranging from political science and political philosophy, to ethics, sociology, behavioral and developmental psychology, and even economics (with the caveat that the entire field of economics needs to be overhauled and reworked from first principles).

It needs to be collaborative, but right now the environment in academia is overly competitive, and that's extremely toxic to human progress. It's a side effect of the commodification of education though.

But if we're just brainstorming, then this is how I'd expand it, in rough outline form.

• Human rights should be central, although elaborating on them fully would likely take more than your hundred word limit. These must be updated to reflect the digital age, and can be added to, but not subtracted from
• • For instance, the Bill of Rights and the Universal Declaration of Human Rights is a good start. But also FDR's proposed Second Bill of Rights, and Richard Stallman's philosophy on digital rights are good launching points for a comprehensive and modernized list.
• Perhaps before elaborating on human rights, because it's arguably more foundational, the overall purpose and role of government needs to be clearly defined. Hobbes's Leviathan still has a lot to say in this respect. People come together to create societies. Societies help distribute the burdens and the fruits of human activity. Governments help organize and regulate human activities within society. And can centralize/pool resources and direct them where they're needed most to improve lives for the members of society. Hence we have taxation. But the allocation of funding from taxpayer dollars must necessarily be a democratic process for this very reason.
• • Governments can also help to direct (again, ideally democratically) collective efforts, such as building a green energy transition (albeit that's a temporal example; a thousand years from now it might be building O'Neill cylinders or terraforming Venus), or maintaining/updating civil infrastructure (such as transportation, communication, utilities, etc.; arguably a more timeless purpose).
• After defining the purpose/role of government, it's structure and function needs to be defined in more detail. Much of this can be more mutable, and subject to change with each constitutional upgrade. But certain key points should at least be set in stone, such as how to ensure checks and balances, how to enforce the rule of law so that it is binding in not just theory but also practice, how to give that enforcement teeth, and maybe a few automatic triggers so that enforcement doesn't depend on fickle human initiative. How to distribute power so that it does not become overly centralized or consolidated.
• • Another important point is how to guarantee adequate representation of constituent demographics (geographically but perhaps also culturally, which may become key to a future transition to a more cosmopolitan/borderless society when the nation-state as a concept ultimately goes the way of the kingdom and the empire (i.e. to the dustbin of history).

Beyond these things, which again I acknowledge is just a rough outline, a brainstorm if you will, and must be elaborated on further by an interdisciplinary collaboration of experts, I believe most of the other details can be more mutable and stand somewhere in between "core constitutional charter" and "temporal legislative law."

Perhaps there could even be a new branch of government whose entire purpose and role is to ensure these middling details stay up to date with the needs of the current century and decade. Perhaps a built-in review/audit every ten years, and a scheduled convention/rework every hundred. Something along those lines.

Thank you for contributing your ideas, I think we're cooking something awesome here. Even if it's ultimately destined to become no more than a message in a bottle cast into the sea. At least the ideas are finding expression in words somewhere.

I don't often find someone willing to brainstorm like this, so really thank you.