More terrible, high-RHD undead!
The dust wight is awful. 16 undead RHD would ruin any monster, but even without most of those this thing would still be underwhelming.
20 strength and two 2d6 slams are awful at this level. DR 5/adamantine is useless when no monster ever feels the need to attack you. 20 ft. land speed is just insulting.
The special abilities are barely any better. Elemental Turning Vulnerability is a blatant weakness, and Crumbling Touch gives a bad power-up to the natural weapons you weren't going to use anyway.
The only mildly interesting ability is Petrifying Cloud, which is at its core a no-action save-or-die. I mean, it's going to have a low saving throw, only works at extremely close range, can only target someone once a day, and needs a turn to take effect, but it's probably the best reason to play a dust wight.
Note, however, that by adding a few RHD a dust wight can advance in size pretty quickly. One RHD makes it Large, with the corresponding +8 strength, -2 dexterity, and +1 natural armor, whereas another two RHD add the same stat adjustments again while boosting AC even further. A 19 RHD Huge creature with 36 strength may not be great, but it's definitely better than the standard dust wight.
Even with the fancy advancement shenanigans, I don't think dust wights are really playable. They struggle to get three attacks per turn (and are flat-out incapable of that if Huge), have terrible stats apart from strength, and their sole save-or-die is hard to protect your team from, very limited, and overly reliant on their low speed to actually get used. -0 LA.