A silly point. They were still a Union in 1776 when we decided we wanted to be independent from them.
The word union was tossed about then, sure. The correct and most widely used term was The British Empire. Just like when people toss around the word Democracy to describe the USA when we are not.
Your opinion that the founding fathers did not care about slavery is discredited by the writings and arguments of that time period. It was a hotly debated subject that was left for future generations to settle...which we did.
https://www.britannica.com/topic/The-Founding-Fathers-and-Slavery-1269536
Let us be clear on this point. Your claim is only valid if you have evidence of it prior to the Declaration of Independence. What they did subsequent to 1776, (The year they actually established the new government) is irrelevant to what was their intent when they established the new government.
You can't justify past actions by claiming future actions made your past actions right.
So do you have any example of the founders great concern over slavery prior to 1776?
Looking at your link, the answer would appear to be "no."
So let's be clear on this point. *WHEN* the founders were actually creating the new government, they did not give a crap about the issue of slavery. As a matter of fact, they forced Jefferson to remove some of his abuses from the Declaration of Independence that cast slavery in a bad light.
Remaining in the Declaration is an actually pro-slavery statement. "He has incited domestic insurrections among us..." meaning slave revolts.
They considered offering freedom to the slaves to be an "abuse" by the British Crown.