I don't believe all Southerners had that desire, but it was a hot political topic. Walker the fillibuster was mentioned earlier. After reading his writings, his goal was to move American style slavery, the institution, into Central America. The debate over slavery was always primarily over the Southern institution of that system. I said, earlier, however, the only realistic sytle of slavery in the unsettled American territories in the 1860s would have been domistic only, not plantation style, or more to the 1860 model, the coop industrial style which was the big money maker.
"Or to act as a counter to the increasing pressure of the northern states in Congress?"
That's a difficult question because slavery, as a Southern institution, gave fuel for Northern political pressure, so by constantly trying to expand slavery, that pressure increased. As I said, the South had lost the parity race, and nothing they could see in the future of the Union gave them comfort to believe they would ever have a dominate or equal political voice. Since there was no way of politically removing slavery in the South, there were not many options left to them. The slavery could not expand to new states; free states were constantly growing; the South was now threated with Northern backed slave insurrections and anti-Constitutional abolitionist. The problem was cultural and geographical sectionalism, and Northern and Southern political and economic rivalry.
"Why was it ever necessary to have an 1820 Missouri Compromise?"
That was when the situation was reversed. The Louisiana Purchased opened up vast suitable lands for statehood, lands already under slave labor; the contested lands in Texas, Arkansas, Missouri, and Louisiana were all protected by law to allow slavery via the cession treaty with France. John Q. Adams believed that the treaty forced them to recognize these areas were open to slave owners. The Missouri Compromise was to protect the North from falling behind in political parity, not the South.
In 1820 the following slave states existed. Although officially many Northern states ended slavery in the 1780s-early 1800s, slavery existed well into the mid 1800s.
Pennsylvania (ended around 1845)
New Jersey (ended 1865)
New Hampshire (ended around 1845)
New York (ended in 1827)
Rhode Island (ended in 1842)
Missouri would join the next year.
Thats 16 out of 24 states in reality, and 12 officially.