As you say, playing in the dunes of Chebbi and Chegaga is rather unsatisfying although you can cross either of these ergs, especially Chebbi, anytime, making it as hard for yourself as you like. Driving and especially riding through true ergs is hard work and dangerous and I would say even a bit monotonous - but some like that challenge.
Morocco does not have any extensive areas of sand sheet as you find in Algeria for example. Knowing both places, route MS6 best evokes the big routes over the border but is a far cry from the real thing. This is Morocko, dominated by the Atlas and so best known for it's mountain and canyon routes, just as Tunisia only really has the small dunes of the Grand Erg.
Or go down to Mauritania - plenty of sand there: rail route to Atar, low route Ching to Oudane or Ching down to Tidjikja - and of course Tidj to Nema or off piste Ouadane south to the Nema route.