Quincy of course;)
So I might put him in the category of late Founder.
Even John Adams was in Europe while the Constitution was being written. Benjamin Franklin was recalled and was able to participate in crafting the Constitution. Jefferson was also in Europe with Adams and missed out on crafting the Constitution.
So the set of Founders is diverse, and spans the time of agitating for the Declaration of Independence through the ratifying of the Constitution. John Quincy Adams did play a part in securing lines of credit and building foreign relationships with the European countries, while the famous Founders were back home finalizing the Constitution.
-PJ