Right: the north wasn't fighting about slavery, it was fighting secession.

But the south was seceding about slavery.

The south started the civil war, over slavery.

The confederacy only existed to preserve and expand slavery.

I’ve found this quote from Lincoln to be illustrative.

“If I could save the Union without freeing any slave I would do it, and if I could save it by freeing all the slaves I would do it; and if I could save it by freeing some and leaving others alone I would also do that. What I do about slavery, and the colored race, I do because I believe it helps to save the Union; and what I forbear, I forbear because I do not believe it would help to save the Union.”

I also find the fact that the Emancipation Proclamation did not free slaves living in border States, but only States that seceded to corroborate this.

That being said, Lincoln had long been know to oppose slavery and supported its abolition.

Read the Confederate constitution and that’ll put this debate to rest.

Yes, the reasons lie somewhere between "it wasn't about it" and "it was only about it". Slavery was a major issue of course but a deeper feeling of cultural separation was under it. The nation back then wasn't nearly as federal as it is now, it was much more a collection of states and people felt that way. Gen Lee always said his loyalty was first to Virginia, he didn't say the US and not the Confederacy. The north was much more industrialized and the south much more rural. Also slaves were expensive and you had a southern elite of wealthy landowners who owned the vast majority of slaves who had much more to lose from abolition than the average poor white person. In fact West Virginia broke away and Tennessee remained mostly neutral because the people in the mountainous areas rarely had slaves. When states became free or slave then it became a matter of whether new states should be free or slave states, further fanning the fires. "Bleeding Kansas" was a mini civil war before the civil war. The wealthy southern landowners saw every free state as a step towards abolition, others saw it as a threat to state's rights by an increasingly powerful federal gov. Ironically the cotton gin actually increased the demand for slaves instead of reducing it.

The war wasn't always popular in the north, especially early on and you had vicious draft riots in NYC and a Massachusetts unit was viciously attacked while passing through Baltimore.

I'll add to your comment on the Emancipation Proclamation that Lincoln also wanted to make the war more than just about reuniting the nation and for a higher cause. He also wanted to make it clear to the now abolitionist British that siding with the Confederates (the British were big consumers of southern cotton) would put them on the wrong side.

Nonono. It was about Cotton Picking Rights.

The Civil War didn't begin over slavery but the South seceded over slavery. It's complex but yeah Lincoln was originally ambivalent towards abolition in 1861.

This is a bad take. The trend in the US and elsewhere was abolition. One reason for secession was the South could see the writing on the wall. Lincoln's election being the most obvious sign.

We could imagine the South losing the war and keeping slavery, but only for a short while.