I would rather stick red hot needles in my eyes than cycle the NC 500, seriously. Five years ago, maybe yes. But now all we hear is stories of the single track sections being blocked by numpties in motorhomes trying to pass each other in opposite directions; people using the passing places as parking spaces, sports car types trying to do it in a day, as well as your normal traffic, motorbikes, tractors, buses, lorries, etc..
Now I'm not sure I should be contributing to publicising this so far reasonably well kept secret, but Galloway Forest Park in south west Scotland would be my preference for cycling. Someone has come up with a 300 mile cycle route which might be worth consideration.
The SW 300.
Edit to add... Having had a look at that route in more detail, there are sections of it which I would NOT be happy cycling. A76 from Kirkconnel to Cumnock would be one of them. Especially when there are alternative minor roads available, apart from a 3 mile section north of Kirkconnel. Even that is a bit hairy, I have done it twice cycling to Dumfries. A76 is a busy single carriageway trunk road with a lot of HGV traffic.
Same applies to the A77 section from Girvan to Stranraer, I would find an alternative to that.
To summarise, D&G is good cycling country but avoid the A76, A77, and especially the A75 (major route for Northern Ireland bound HGV's via ferries out of Stranraer). I don't understand why they have included them when there are much better routes available.