I would still recommend Cairn Gorm and Ben Macdui because they are easy to access, and Ptarmigan are hit and miss anyway.
Take the highest parking lot and walk from there. I have seen Ring Ouzel on that parking lot in the past. On the sub-arctic plateau that is Cairn Gorm and Ben Macdui, I have seen Ptarmigan there several times, including with chicks. You can also see Dotterel and Snow Bunting there. For Dotterel, listen to the calls in the grassy areas. Snow bunting can be quite conspicuous when flying around and singing. Confiding too! When I was eating a snack on the summit of Ben Macdui, a pair landed just 2 metres away from where I was sitting.
As for the Ptarmigans, they are roaming around on the highest areas of this big plateau. I have no specific tips where to look. The best way is probably to just roam around yourself on this rocky plateau and sooner or later you'll come across them. Also listen for their calls.
I have climbed 182 of the 282 Munroes in Scotland so far, and I regularly came across Ptarmigans. They aren't particularly rare. Usually I see them on or very near the summits.
View attachment 1577585