Personally, I consider the Constitution to be even better. Too bad the federal government no longer follows it. :-(
However, I don't see how your quotes deal with the issue at hand. (Other than that blacks, in any state or portion thereof, where they were in a majority, would be considered by the Declaration of Independence to have a right to secede from the U.S.)
I understood you to be quoting the part that you quoted as some sort of vague support for the notion that you and your neighbors may secede from our Union at will and without risk.
The first passage from the Declaration that I quoted was to suggest that there existed in the minds of the authors of that document certain important values the preservation of which might warrant the drastic step of declaring one's independence of an existing government. You might find some of those values relevant to the institution of slavery.
I quoted the passage from the bottom of the Declaration as sort of a reminder that the authors of that document did not delude themselves into thinking that one could simply declare themselves independent of an existing government without great struggle or risk. Revolutions and rebellions are only for the most serious of players.