You ought to try reading what they themselves said about secession and slavery.
Which, I would add for emphasis, were two separate issues -- related like two people on either end of a telephone line, but not organically or causally related. Read the list of their complaints against the Northern-dominated Congress and get some idea of what they thought their issues were. They were candid, lucid, and not at all reticent.And knight me no conspiratorial golden circles, either.
If there were any conspiracies, they were in the North, where Lincoln spent March and the first half of April, 1861, all hugger-mugger in secret meetings with six, or five, or nine Yankee governors, holding war councils with his stoutest gubernatorial supporters, men whose support would be essential as he prepared to reach around the Congress and turn the United States into a military empire.
But the departing States' complaints varied from place to place; Texas, for example, had a serious complaint about Comanche raids and general insecurity and Congress's deliberate refusal to appropriate funds and send troops to help defend the State in accordance with Congress's constitutional duty. Mississippi's declaration of causes was different, and South Carolina's (search under "Robert Rhett") showed still other causes, including the disappearance of the South Carolinian shipbuilding industry because, they said, of favorable treatment Congress had given New England shipyards.
The causes varied, they all mentioned slavery specifically because Northern politicians had made the slavery issue their club wherewith to beat the South into submission politically, dividing the country into a greater part (theirs) and a lesser (the South), and setting the table for a huge Yankee Thanksgiving at Southerners' expense.
Economic exploitation of one region by another more politically powerful -- what country hasn't seen that? But this was America, and (according to "Publius" in The Federalist), some States weren't supposed to be able to eat the other States' babies.
That's why the South left.
Oh, and there's a reason the South's economy didn't even get up on its knees until World War II, and it wasn't because Southerners "talllkkk slooowwww" or are stupid. It was due to Northern businesses' "unlocking the value", to use a phrase bandied about much later by Wall Street raiders, of their strategic middleman position w/r/t Southern economic flows.
In other words, the South was on her back because Northern business interests were sitting on her chest from 1865 to 1945, and then some.
So, I took your advice and searched under "Robert Rhett". I came up with the below. Is this the same Robert Rhett you are referring to?
Proponents argued that the Golden Circle would bring together jurisdictions that depended on slavery. The Knights of the Golden Circle was the U.S. organization formed to promote and help create the Pan-American union of states. It was organized in 1854 by George W. L. Bickley, a Virginia-born doctor, editor, and adventurer living in Cincinnati, Ohio. Membership increased slowly until 1859 and reached its height in 1860. The membership, scattered from New York to California and into Latin America, was never large. Some Knights of the Golden Circle active in northern states, such as Illinois, were accused of anti-Union activities after the Civil War began. Robert Barnwell Rhett, called by some the "father of secession", said a few days after Lincoln's election: "We will expand, as our growth and civilization shall demand - over Mexico - over the isles of the sea - over the far-off Southern tropics - until we shall establish a great Confederation of Republics - the greatest, freest and most useful the world has ever seen."