Here I agree with you. Of course they wanted more land and more people because from the combination of the two, more power grows.
Some individuals? No doubt. The states as a whole? They made no claim upon any US territories. They didn't start trying to seize any of them until AFTER Lincoln started the war. Once a war was on, of course they then tried to seize some of the Western territories.
Confederates "tried to seize" whatever they thought they could -- from New Mexico & Oklahoma to Missouri, Kentucky & Maryland.
They also challenged Unionists in Kansas & West Virginia while sending Confederate forces into Pennsylvania, Ohio & Indiana.
Confederate guerillas operated from California & Colorado to Vermont(!)
So any suggestions that Confederates "just wanted to be left alone" are pure nonsense.
It has long been my belief that if the Confederacy had been allowed to become stable, all the outlying territories and many of the states then part of the Union would have gone over to it. The five border states would have of course gone over to the Confederacy. Several of them came close anyways, and were only prevented by use of Federal forces to stop them.