When you say "all-terrain," do you really mean it? Because there is a world of difference between a bicycle that can handle rutted Siberian tracks during a blinding snowstorm; and one that will allow you, in a pinch, to safely travel on gravel for 20 minutes. For the latter, any decent touring bike will do. I once took a very long "shortcut" on a mountainous, poorly maintained, logging road on 23 mm slicks. I was fine. But I would not go anywhere near powdery sand on that bike! (I tried it, so can assure you that skinny tires and sand do not mix!)

A bomb-proof bike is possible, but can you make do with something that will work under the conditions you will actually encounter? (And if that means avoiding climbing sand dunes and crossing raging rivers, so be it!)

another bike:

my surly pugsley

https://forums.mtbr.com/showthread.php?t=179026