You are quite correct, for the Founders. One can accept slavery as an evil while still recognizing that it is impossible to do away with at the moment for practical reasons. That was the position of most, though perhaps not all, of the Founders.
The problem arose later, when southern fire-eaters starting with Calhoun began declaring slavery to be a positive good. At that point one cannot believe simultaneously in slavery and in the principles of the Declaration without demoting the slave, and by implication the black man, from being a full man.
I don’t know why you would want to try to argue this point. Taney was extraordinarily clear on the point in his Dred Scott decision.
“They had for more than a century before been regarded as beings of an inferior order, and altogether unfit to associate with the white race either in social or political relations, and so far inferior that they had no rights which the white man was bound to respect, and that the negro might justly and lawfully be reduced to slavery for his benefit. He was bought and sold, and treated as an ordinary article of merchandise and traffic whenever a profit could be made by it.”
If that isn’t racism being used to justify slavery, I don’t know what would be.
Wrong again. The problem began when more and more people began to understand that black people were human beings and indeed worthy of living as free as any other man.