The 10th Amendment does not authorize secession "at pleasure."
Nor, so far as I can find, did any actual Founder speak in favor of secession "at pleasure."
Again, I invite you to review your vast collection of Founders' quotes to see which ones spoke out specifically for a "right of secession at pleasure."
The crux of the matter is sovereignty. Who are the sovereign?
Excerpt of letter from Shuyler Colfax to Lincoln 02/01/1861(emphasis mine):
...”For myself, I can say that while willing to vote for a Constitutional provision against interfering with Slavery in the States, which is merely declaratory of what I regard as the Constitution now, & to restore the Mo. Compromise by law, (but not to put any lines in the Constitution) I have an insuperable repugnance to making New Mexico a State, when,with this sovereignty, she may secede the next month as she cannot now And I look upon the Convention here next week with great distrust.”
IOW - “with this sovereignty,” (as a state) “...she may secede the next month” (or “at will”) “...as she cannot now” (as a territory).
Indiana Congressman Colfax (1855 - 1869), speaker of the house (1863 - 1869), and 17th VP, unequivocally acknowledges that the states had the right to secede whenever they wanted.
Hat tip, PA:)