Have done that section in the past on two occasions, although from south to north. The A816 between Oban and Kilmartin (slightly north of Lochgilphead) looks like it
ought to be busy as some have suggested; we didn't find that on either occasion, even in late July. There was traffic of course, but nowhere near what you might find on a road with a similar designation south of the border. Maybe we were lucky, or maybe things have changed in the intervening ten years. I would echo the sentiments that others have expressed though - depending on when you're able to travel, current circumstances make it likely things will be much busier this summer. If you're prepared to live with that possibility though, the ride is a corker.
If I were to do it again, simply for variety, I would choose NCN Route 78 - I think you considered this as a possibility? The road down the west side of Loch Awe looks wonderfully quiet and, from a
quick mooch on Streetview, has a decent surface.