I would go up the A82 to Inverness and only join the A9 there. But I live near Glasgow and that end of the road I lived in a village on a single track road and it is amazing how many people do not prepare for them or know they will encounter them on their travels. I think autumn would be a good idea. The weather has been good then for the past few years and if you stop at a windy west coast place, the midges will not be too bad. I’d use the A9/A939 via Braemar on the way up to Inverness. On the way back just follow the West Coast south from Ullapool all the way to Ardelve/Dornie next to Eilean Donan Castle (of Highlander movie fame) via Gairloch and Applecross. The A87 and A82 will then take you back to Glasgow via Glen Coe