Plenty of good suggestions above. Crask inn worth a stop and you ca camp on the lawn if it suits your timings. After that I like the Strathnaver road and the one down Loch Loyal to Tongue which your route misses out (I guess you chose the Helmsdale way to get to Douneray though, and it’s not bad either). If time allows add some side trips up those other roads though – interesting history and very quiet. North coast beaches are great and easy access to several that rival many more famous ones on the west so plan to stop often. On the west, a trip out to Handa is a good detour at this time of year if you’re into wildlife. Further south the coast is hillier than inland but for my money more interesting riding, better camping (and obviously great beaches). Stac Pollaidh is a great little scramble on the roadside, well worth it for the views in all directions.After Garve the falls of Shin are worth stopping off at, as is Black Isle brewery a mile or two off route. If you have time to kill then continue out to the east coast instead of straight over to Kessock, loads of small towns worth exploring.
Kinlochewe-Kyle is potentially a good idea, but the main road west-east isn’t that bad as there’s only really a short section after Garve with poor sightlines where you’ll be going slowly, most of the rest you’ll have decent speed (downhill) or visibility. If you don’t fancy it or get delayed then another option is a bike bus that runs Inverness-Durness daily in summer.
FWIW me and ms Bajsykel did pretty much this route in reverse from Inverness over three and a half days with several stops each day. A week would be much more enjoyable as there is so much that you have to miss rushing like we did.