All you need for constitutional reform is a 50% majority on a referendum. That seems way too easy.
I don't agree. The U.S. Senate is very powerful; those restrictions aren't very broad.
> the simple 50% majority is by design
It depends what you mean. The filibuster, which requires 60% majority, has been part of the Senate since around the 1840s; clearly many generations of Senators thought it was important and intended that it continue.
It is ... it's still there, just not for confirmations of appointments by the executive branch.
It still is. Filibusters have been banned only for confirmations of appointments by the President.