You are correct that the US Constitution is poorly suited to governing a nation of ~1/3 billion people in the modern world. Unfortunately, the current political environment make fixing things impossible.
https://en.wikipedia.org/wiki/French_Republics
Admittedly, far more an exception than an ideal to emulate.
The other "typical case" is countries liberated from military occupation...
The US is pretty much alone in that respect. So it’s easy to say that a lot of terrible regimes were enabled by constitutional change. Most of them were. But so were most of the regimes that are much more democratic than the US.
This mirrors the weird fetish that some American Christian’s seem to have with their bible. These things were written by humans. So, of course they are imperfect and can be discussed and improved upon.
France went through counter-revolution, counter-counter revolution, empire, revolution, restoration, revolution, re-restoration, revolution, empire, revolution, republic, WWII, and republic. The US never came anywhere close this kind of collective trauma, even at the worst times of the secession war. Most countries are not protected by two ocean, a small border in the south and a compliant neighbour in the north. Most countries don’t have the luxury of hanging on a thoroughly outdated bit of political philosophy.
Which is exactly why this is such a terrible decision.