Build rather than buy perhaps? FWIW ..
Humboldt Kinetic sculpture racers started using ATV wheels.
Dan Hanebrink used to make his extreme terrain big bikes with those..
wonder if he still does, belt drive X 2,

the disc mount on Rohloff hubs is the same 4 bolt pattern for a chainring,
and people have already used that function as a jackshaft drive , outside of a wheel.
[Belt cog instead of chainring , then ]

The Arcata-Eureka folks back in the 90's, also used
a strapped together set of 5 rims, the fattest tire in the center ,
to only ride on it, being a larger diameter.. on pavement was also used.


yea a belt drive would be better, still have abrasives all around you
and so washing down the sand , from the cog wheels, cleaning he bike,
will still be very wise maintenance..