I don't know anything about this really but I would have thought it would have been the opposite: emperors who hold power over many disparate cultures would want a polytheistic religion so they can subsume other cultures' gods into their own pantheons, therefore saying "all gods are real, we just didn't know about yours until we conquered you"
Which would also fit in with why Jews and Christians were discriminated against: being monotheists isn't conducive to unity across an empire if you have one group of people claiming their god is the real one and all the others are not only fake, but it's actually a sin to believe in them
You have correctly identified where pantheons come from. The term for these religions is "syncretist". Each tribe, village, city-state has its god or gods that join in, or are merged into others already there (some get lots of names).
Then when the emperor wants to solidify his power, he replaces local kings with his own people, and identifies himself with a chief god.
Akhenaten, Tut's father, tried it in Egypt, with Aten as big cheese god. It didn't work out (probably) because the priesthood turned out still too powerful.
But that goal probably is a big part of why Rome went Christian.
Which would also fit in with why Jews and Christians were discriminated against: being monotheists isn't conducive to unity across an empire if you have one group of people claiming their god is the real one and all the others are not only fake, but it's actually a sin to believe in them