this episode is about the nationalpark. these roads are in a extraordenary bad condition even in the dry season. I did it with a 1150gs and two persons, a second rider is recommended to lift your bike ones in a while
In the rain season i drove only with a 4x4 and could not imagine riding it! Deep mud, rain, snowfall, hard wind...
But as i mentioned and did you can ride it in the dry season, the east route around laguna colorada is easier than the west one, the west one was very sandy!
to your question: i haven´t biked the route you are refering to, only from tupiza to uyuni. The roads are bad in the dry season, actually every diffiucult underground you can imagine you will find. Also deep muddy river crossings.
In the rain season it must be a hell of a challenge... i would recomend doing it in the dry season. Also because you wanna visit the nationalpark, it´s amazing there!
As another option you could ride from tupiza to potosi and than to uyuni.
Both parts are with asphalt and easy riding.
Of course the experience will be different.
If you are at Salar de Uyuni in rain season, always ask the locals with their 4x4s where to go off the lake! You can drown otherwise and they know the safe exits!!!