The founding fathers did the best they could in the political climate of the times. Advocating "All men are created equal" in the 18th century was groundbreaking
However, trying to equate the needs of an 18th century impoverished America to a 21st century Superpower is foolhardy. Each generation must establish the type of government that best meets the needs of their era (within the framework of the Constitution)
Thomas Jefferson said it best..
"I am not an advocate for frequent changes in laws and constitutions, but laws and institutions must go hand in hand with the progress of the human mind. As that becomes more developed, more enlightened, as new discoveries are made, new truths discovered and manners and opinions change, with the change of circumstances, institutions must advance also to keep pace with the times. We might as well require a man to wear still the coat which fitted him when a boy as civilized society to remain ever under the regimen of their barbarous ancestors."
You went with the liberal amended version of that quotation that is used dishonestly to present an opinion that Jefferson did not hold. I'm not saying you are dishonest, but those who amended it were.
The original version:
"I am certainly not an advocate for for frequent and untried changes in laws and constitutions. I think moderate imperfections had better be borne with; because, when once known, we accommodate ourselves to them, and find practical means of correcting their ill effects. But I know also, that laws and institutions must go hand in hand with the progress of the human mind. As that becomes more developed, more enlightened, as new discoveries are made, new truths disclosed, and manners and opinions change with the change of circumstances, institutions must advance also, and keep pace with the times. We might as well require a man to wear still the coat which fitted him when a boy, as civilized society to remain ever under the regimen of their barbarous ancestors." - Jefferson to H. Tompkinson (AKA Samuel Kercheval), July 12, 1816[10]
Thus a provision to amend the Constitution was included in the original document. And to assume that those who signed the Constitution were advocates of slavery is to totally dismiss the very clear record they left behind. The vast majority of the signers did not advocate slavery.