How about the proceedings from the 1856 Republican National Convention, during which the first Republican platform was debated and developed?
https://archive.org/details/proceedingsoffir00repu/page/n8/mode/1up?view=theater
I took some time to skim through the 1856 convention proceedings, and suggest you do the same. I don't think a reasonable person could read this and conclude anything other than the formative Republicans' goal was to end slavery in the USA, but chose to limit their platform to the expansion of slavery in order to garner popular support and to avoid overstepping the States' rights by demanding a federal abolition of slavery. One relevant excerpt from the proceedings:
P. 16-17, introductory speech by Robert Emmet, President of the Republican Convention: "Without exception, all the great men of that day [the founding of the republic] foresaw and predicted that slavery, although it could not be summarily and suddenly abolished, would die out in this country. All acknowledged that it was an evil. All acknowledged that it was the policy of the country gradually to get rid of it. That was the policy of that day. That policy led to the adoption of what was called the Missouri Compromise. [...] We could not make all the Southern States free at once. We had then to draw a line; and let it be understood that it was by that line - the Missouri Compromise - slavery was to be limited, and that it should never extend north of it."
“I don’t think a reasonable person could read this and conclude anything other than the formative Republicans’ goal was to end slavery in the USA, but chose to limit their platform to the expansion of slavery in order to garner popular support and to avoid overstepping the States’ rights by demanding a federal abolition of slavery.”
That is an interesting comment: the official, published platform was limited to the prevention of slavery expansion, while every reasonable person knew their real goal was the overthrow of constitutional slavery.
I have to note no Republican introduced a proposed constitutional amendment to abolish slavery until after the war started. I wonder how they expected to overthrow constitutional slavery without introducing a constitutional amendment?
Perhaps the unspoken intention - that every reasonable person knew - was to use extra-constitutional methods to overthrow constitutional slavery.