In some ways, this is not dissimilar to the American Civil War. All the Confederacy had to do was not lose. But the similarity ends there as the Confederacy did not get the international support from countries that were major cotton importers (primarily the UK) that they had planned on. The stumbling block being the chattel slavery practiced in Confederate states was seen as a moral wrong by the middle/lower classes in the UK after Wilburforce. But the 'why' is to the side. Also, similarly to the North, the RF has a significantly larger population. During the Civil war, that translated to larger Union armies, but look at the RF demographics. It's not clear they have the conscriptable-age population to successfully invade - and they definitely don't have it for occupation.

So far, Ukraine seems to be doing a decent job of not losing - and they are getting international support (it seems pretty clear why they are getting it). So the RF's focus is on discouraging that support with as many levers as they can use.