There are many synonyms for the phrase "staid on treadmill" that can convey a similar meaning. Some possible options include "stuck in a rut" or "in a monotonous routine." Other alternatives might include "trapped in a cycle" or "stagnant in one's progress." Another possibility is "going through the motions," which implies a lack of engagement or enthusiasm. Ultimately, the choice of synonym will depend on the context and tone of the writing, as well as the specific image the author wishes to convey. However, all of these synonyms share the common implication of being stuck in a routine and lacking progress or excitement.