Rhett lays out the economic case. He details how the Southern states as of that time were in exactly the same position vis a vis the Northern states as the 13 colonies were vis a vis Great Britain in 1775. He goes on at length about how the high tariffs are harmful to the South's economy and how the North uses its congressional majority to lavish on itself the vast majority of the money raised by tariffs paid for overwhelmingly by Southerners.
The fact that he addressed it to the other slaveholding states....overwhelmingly the other states with economies primarily based on producing cash crops for export in no way undercuts the economic arguments.
The case would have been exactly the same had those states abolished slavery and produced cash crops for export via a sharecropping system which they adopted after the war.
You are right, as far as you go about halfway through Rhetts address. If you read the second half of Rhetts address, you find it talks almost exclusively about slavery, and not taxes or tariffs (I thought I had made that point in my original response). To summarize, it appears that the second half was spent a lot of time talking about how the South was a collection of slaveholding states, and that they never would have agreed to Union if they thought that the North would try to abolish slavery. I assume you have read the whole (not just the first 8 paragraphs) of the address. Im sure you have seen the last line, which reads We ask you to join us, in forming a Confederacy of Slaveholding States. It doesnt say join us in forming a Confederacy based on low tariffs, or low taxes. It appears that the definition of a Slaveholding Confederacy is the important definition, not taxes or tariffs. I am sure I am misreading the last half of Rhetts address, and the constant references to slavery are somehow unimportant, and I am sure you will respond to let me know exactly how I am misreading it.
To be clear, it is not my contention that slavery was the only reason the South chose to secede. It is however the primary reason.