Likewise, Ben Franklin being a founding father doesn't somehow balance the scales in that regard. The existence of the United States doesn't retroactively make slavery ok.
The Founders saw fit to abolish the nobility, and disassociated religion from government. We've forgotten what an enormous leap both of those were (each of the colonies had an establishment of religion). I've seen endless documentaries on the founding of the country, and there's no mention of either of these.
Half of the colonies did abolish slavery. But, again, if it was demanded of the other half, the Union would never have happened.
Jefferson, Franklin, and Washington all held slaves.
I am certainly not advocating slavery. I'm merely pointing out that people should be judged on the morality of their times, not today's morality. If we judged past people by our modern standards, they would all be condemned.
Also, if we must judge by the morality of the times, we must also view abolishing the nobility and separating church from state to be immoral and unchristian. You seem to want to avoid a "modern moral framework" when it would tarnish the reputation of a founding father, but embrace it otherwise. You're not advocating slavery, but you don't want to judge slaveowners harshly.
It's perfectly fine to acknowledge the good that people do, while also acknowledging the evil. Jefferson, Franklin, and Washington all held slaves, and they were all evil for doing so. If we judged past people by our modern standards, they would all be condemned, and if we believe at all in some form of moral progress (which you must, since you bring up American freedom and the establishment of a secular republic as virtues) then we can condemn them all for some things.