It is always better not to elevate any religion to the level of state policy.

Well, you're talking about one thing, and I'm talking about another. Okay, let's agree with that statement and say that in the USSR, the religion of "atheism" was elevated to the level of state policy, while in the Empire, no religion was imposed at all: if you want, be an atheist; if you want, be an agnostic; if you want, be Orthodox; if you want, be a Muslim. And also Catholics, Protestants, and many others. And although the official religion was Orthodoxy, it was just a document; there were representatives of a huge number of religions in the country, and everyone felt perfectly comfortable with the philosophy that was closest to them. And you know very well what it was like in the Soviet Union in this regard.
----------

different cultures simply cannot unite people

The Russian Empire existed for two hundred years, and it collapsed only due to the activities of our own Russian revolutionaries. What united them? A common idea? No. Culture? Definitely not. Common sense, the understanding that together we are stronger, a common sense of belonging to a great state that was stronger than petty national ambitions? Perhaps, at least I can't give any other explanation.