From what I've read, the Russian army attacked Kyiv, because they believed the Ukrainian government would fold to Russia's demands (no NATO, recognition of Crimea being a part of Russia as well as autonomy for Donetsk and Luhansk People's Republics).
Once they realized that the Ukrainian government made the decision not to negotiate for peace and fight instead, they withdrew, regrouped, changed their strategy and now we have what we have.
During the initial invasion, the Russians only committed around 140-160,000 troops. It doesn't take a military genius to acknowledge the fact that you can't conquer the largest country in Europe with just 140-160,000 troops. So I don't believe that Russia intended on conquering all of Ukraine.
As for Odessa, Odessa has significant strategic importance. It's a port. I think it's the only large port that Ukraine has left. So strategically it would make sense for Russia to try and capture that. Capturing Odessa would cut off Ukraine from the Black Sea completely. And ensure that NATO can't station their navy there in the future.
