Certainly the Loch Builg/ Upper Glen Gairn area is very pleasant walking, I often take the tent up to the ruins at Builg Lodge and spend some time wandering about the area. There is an excellent stalkers path along the northern bank accessed by going through the deer fence at an unusual type of gate at NJ 190023. Don't rely on the footbridge marked on some maps at NJ 179009, it doesn't exist.

Are you locked into heading backwards towards Braemar?

Going over the watershed between Glen Gairn and Glen Quoich you have the option of following the Quoich all the way to the Linn of Quoich or cutting off over the shoulder directly towards Mar Lodge but either way you are faced with fording the Quoich a couple of times, which given you are considering the route as a FWA could possibly be difficult. The crossings at NO 111958 and NO 079947 have both filled my boots. An alternative is to use the route down Gleann an t-Slugain. It may seem counterintuitive but doesn't have any serious fords and the route down through the floor of the glen (as opposed to the landrover track that follows a higher route) is very attractive, and isn't an awful lot further than sticking with the Quoich. There is some excellent wild camping in the narrow part of the Glen (and a secret howff if you can find out or work out where it is). If the weather is really foul it might also be a bit less exposed.

Just an idea to consider.

Thank you all, appreciate the input!