I suspect getting a bit further south than Tain on day 2 would allow you to do the Lecht and Glenshee passes as you head south on Day 3(some of the best roads in Britain IMHO).
Once you are on the A9 heading south on day 2 you can eat up miles as it's a proper A road. It can be busy but it's easy riding compared to the North and West coast sections.
I’d agree with that. Day 1 looks right, and the next 2 days are easily extended a bit, i think.
Not too much accommodation around Lochinver, though.
AL