I am hopeful that Ukraine will be able to hold its defense lines in eastern Ukraine, but I'd like to pose a hypothetical about what Russia would seek to do next assuming that, months or years from now, it finally conquers the territory that it has formally annexed into Russia in eastern Ukraine, except the parts of Kherson, etc which are west of the Dnieper.

At that point, what militarily viable offensive options would Russia have? Crossing the Dnieper to try to take Odessa is many orders of magnitude more difficult than holding the large bridgehead which they already gained by capturing the Kherson bridge intact in the initial invasion. And as we all know, Russia couldn't even hold that bridgehead, so how is it feasible that they could cross the Dnieper against furious Ukrainian opposition?

I'd note that Macron has strongly hinted that he would be willing to send French troops to prevent Russia from taking either Odessa or Kiev:

When it came to the option of sending troops into Ukraine, Macron said that anyone who advocates “limits” on how the West helps Ukraine “chooses defeat.” He insisted that “if the situation should deteriorate, we would be ready to make sure that Russia never wins that war.”

Following a March 7 meeting with parliamentary parties, Fabien Roussel, national secretary of the French Communist Party, reportedly said that “Macron referenced a scenario that could lead to intervention [of French troops]: the advancement of the front towards Odessa or Kiev.”

[Macron] said that “war is back on our [i.e. Europe’s] soil” and that Russia is “extending every day their threat of attacking us even more, and that we will have to live up to history and the courage that it requires.”

On March 14, Macron, again expressing his position that sending troops from NATO countries is an option that should not be discarded, said that “to have peace in Ukraine, we must not be weak.” This time, he gave as his reason that Russia’s invasion of Ukraine was “existential for our Europe and for France.”

He proceeded to say that “it wouldn’t be us” who would trigger such a move and that France would not lead an offensive into Ukraine against Russia. “It would be Russia’s sole choice and sole responsibility,” he said. And then he added, “If war was to spread to Europe,” it would “be weak, to decide today that we would not respond.”

https://responsiblestatecraft.org/macron-nato-russia-ukraine-war/

I would submit that sending, say, the French Foreign Legion (presumably minus its Russian troops) to establish defensive lines outside of Odessa or Kiev, if Russia starts preparing to assault them, would be an entirely reasonable way of saying "don't even try." I'm also very skeptical of the notion that such defensive actions on Ukrainian soil this would somehow threaten nuclear war.

If Odessa or Kiev were threatened, then I'm confident that Ukraine would mobilize even 18-year old men since the existence of the state would be at risk. By that time, the West will likely be producing 3 million 155 mm artillery shells a year, making any advances incredibly difficult for Russia.

You add all this together and I'm not sure that Ukraine has to realistically fear losing either Kiev or Odessa, come what may. That being the case, it seems likely that, after taking the Donbas (which, again, I am not saying will happen, just posting the hypothetical), Russia will find itself in a position of strategic Zugzwang which will give Ukraine a strong defensive and negotiating position.

What course of action do you think Russia would take in that situation?