The top of macdui is OK, large expanses of big, loose boulders that weren’t really rideable. Between the summit of Macdui and the ski centre is rideable for a section, but then turns into bouldery trials again.
I spoke to one of the guys at Bothy Bikes, he had a kona cowan full susser set up with a dual ring chainset and pikes, but had used it to do similar routes to the ones you’re talking about.
Also, if you’re prepared to go slightly further south, Cairn of Claise, just south of Braemar, is quite accessible, and the descent towards braemar is brilliant fun, in a sort of swooping, loose, rocky landrover track type way.