“stretching only makes the paper dry flat, it doesn’t actually mean the paper wont buckle during painting”

For paper that's only 140 lb like you're using, yes, this is exactly the case.

If you have to have paper that's going to stay completely flat while you're working on it, you're going to have to go heavier, or with watercolor board like this, or allow your paper to dry when it starts to buckle as you're working on it, or dry it with a hairdryer periodically as you work.