ok....I like the idea of its mind being a solid as a rock...with all mental-based saves...
Shouldn't it be Speed 0 ft.?
Updating the Chthon by adding the Mental Stats, Saving proficiencies and removing its:
False Appearance. While the chthon remains motionless, it is indistinguishable from a normal boulder. [?]
Regarding the "a note in the description that they can move very slowly if needed but much too slow to be perceptible in combat" you mentioned earlier, I think we could include that in its Rooted trait.