There is a great difference between immutable and difficult to change.

Do you want to be in a US without the Fourteenth Amendment?

Actually it strictly immutable without any of the Amendments, no 1st, 2nd etc.

I think ratification by 2/3 of states fairly meets the definition of "difficult to change, but not immutable."

That it's been done numerous times in the past suggests it is quite far from immutable.