Would you be thinking about Mashatu Game Reserve?
I’ve only heard good things about the place, and you definitely ride in among the animals as it is a private concession. Nice landscape and not too sandy.
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There’s also a fleet of fat-bikes on the fringes of the Okavango Delta, contact Delta Rain and ask for Gerald. Not too many animals around their base camp though, and not sure if they ride out into wilderness areas. Very sandy.
In your position I think I’d want to do a bit of bike and a bit of something else: consider a few days at Mashatu to get a biking fix then go up to Maun (public transport, buses aren’t too bad… honest) and either a mobile camping tour with the likes of Delta Rain or book direct with Oddballs for a fly-in Okavango experience.