Can I check which loop you’re doing? Or at least which of them you’re planning to do McD from?
I guess you’re doing the enormous 300km one if starting from Blair Atholl….
Depending on the above, if we assume Aviemore as a point of reference, what about going up Cairngorm then across the plateau to McD rather that going Ryovan/Bynack Mor/ fords of Avon…
Highland man’s suggestion here is really good. Nice cycle as far as Sron Riach and then a reasonable push for most of the way up. Ie nice grassy wide path with no rocks and heather to catch wheels. ( Obviously the top section is scree hell, but you can’t have everything)
Obviously this would be more of a height deviation than your OP