I would get the 305/70R19.5 if you can make it work with room and gearing.
Your weight will always be more than you think and the 305 XZA in 18 ply rating has almost 500kg more load capacity than the 285 XDE+. It also has almost 6% bigger footprint which will make a difference on sand.
Are you going to fit the same tires on front, I hope so in a 4X4! Will they fit at full lock?
My Unimog "gained" 2.6 tons in the rear between pickup at Unicat with empty tanks and current state with full tanks and everything it carries.
Unimog U500 with Unicat camper; diesel BMW X5 35d, diesel BJ40 Landcruiser and diesel M37