Here's the big bike bypass around the sand sections on the NE side of the mountain.
This is a dual sport route and will of course have some soft or rocky or whatever spots, but this will route you around the worst of the sand. Conditions may vary with season and traffic but there's nothing here I wouldn't ride on my 990 loaded.
This track picks up at the intersection of FR31 and FR19 and takes you into Weed. After Weed, there's no splits for the big bikes as none are needed.
PS-sections like this are the reason for the bypass. If this looks fun, take the original route or the Military pass alternate...