Imperialism isn't a policy that states have, it's a global system of extraction from poorer countries into the countries that control most of the finance capital on Earth, it's not "big country goes to war with smaller country." Russia does not control global finance, global finance will pretty much always be their enemy. A stronger Russia (if it was still a semi-periphery country) probably would be better than a strong US, you're underestimating just how terrible the US is, and you're comparing apples to oranges, because the US is a country in the imperial core and its bourgeoisie control capital on an international scale, the bourgeoisie of Russia have almost no power outside of their country.

The concept of imperialism that you're using was first used popularly by Lenin, and it was an economic definition that described a global system. You should learn what unequal exchange is and what super-exploitation is, I think they're good concepts. Also read Neocolonialism by Kwame Nkrumah.