Is there a required or recommended minimum distance of an HVAC duct from an exterior wall?
There is no minimum for running hvac near an exterior wall.
In fact you can run hvac outside (my old house had hvac under slab) and you can certainly run hvac in exterior walls.   There are extra insulation precautions and some loss of energy in these cases but you have to do what you have to do.
Your case isn't as extreme.   In fact it isn't extreme at all because your hvac is inside of the insulated bubble albeit closer to the edge of the bubble.   Meaning that the vents themselves will aid in the cooling and the heating of said bubble.
Addendum:  Given the conversation about the lines in the attic I want to make two points.

If you live in a hot summer/cold winter climate I would not run flex lines in the attic - rigid all the way.

You need to WRAP your lines in the attic, not just bury them in blown in.   You must treat an attic extension as you were running the lines outside - because basically they are.


And obviously we want the vents placed on the exterior because that is the coldest/hottest part of the house and the area that needs the most temperature control.   No issue with your install.