pretty bad flies in the Highlands in july to Sept if there's no wind!! fortunately that's not that often. While riding Rte35 one summer they were so bad I was really annoyed that I was riding with a following wind, it meant the little buggers could keep up
The wind tends to get a little stronger mid morning to early evening, I would often ride until about lunchtime and then stop to eat for a while if the wind was particularly strong, to I had made it to a fuel/fuel station.
I preferred late in the summer as the river crossings are usually lower if your doing the interior roads, and of course you may get fewer tourists.
something I noticed on my first trip, which also dictated how I planned my days if visiting a well known site/attraction od that most folk work on a 9to5 timescale! they usually have breakfast and then drive to where ever in the rental or coach tour, then head back around 5:00/6:00 for dinner in the hotel or local restaurant. as a consequence you often get the places practically to yourself effectively. Given you have pretty much 22hrs of daylight its no problem at all from that point of view.