Russia could just blockade the city and starve out the resistance. If Russia blockaded Odessa, there isn't much Ukraine could do break the blockade. That strategy is how Russians won at Stalingrad. They didn't need to drive the Germans out, they merely surrounded them and starved them out. I see no reason that wouldn't work here, assuming they can cut off supplies from the west or from the sea.

Despite all the propaganda about how poorly Russia is doing, they still control significant parts of Ukraine, and they managed to cost the West billions of dollars in humanitarian aid. They displaced about 4 million people, and when they finally pull out, the West will provide Ukraine billions of dollars to rebuild. In the end, they will have weakened their enemy, and they've sent a clear message to Ukraine that joining NATO is not acceptable. Don't be surprised is a commitment to exclude Ukraine from NATO is a part of any negotiated pullout from Ukraine -- something that NATO could have agreed to before the war started to avert the whole thing.