We don't know the strategy and the plan. We might, even maybe reasonably, deduce that Russian leadership thought that Ucrain would fall more quickly. And that some form of stable solution would be the result.
But most people don't only have one plan to reach a longer term strategic goal. So thinking of "the Russians" as rational players (and taking a lot of potentially missing information into account due to every body only hearing their own side's propaganda) we could at least ask ourselves, if the plan was to overthrow the Ucrain quickly, to probably secure the Donbas and also probably secure the water resources for the Crimean peninsula as a strategic necessity, what course of action remains for Putin now that at least the quickness of the action failed. While there are already people reporting that Russia secured most of the Donbas and the water resources for the Crimean. But how much is propaganda there, I don't know.
So Putin needs to appear as the winner in the end. At least internally. He needs to sell this to his power base and his own ego as well probably.
The west wants/needs to see the Ucrain at least as partial winner. And the sanctions to have worked (because that was sold as solution to the voters).
Can there be a way to end the killing, save thousands of lives and reach these goals? I don't know. But I would be at least hesitant to call Putin irrational. Maybe that is valid if one defines the criteria of evaluation up front. What defines 'rational'. Because I suggest that within Putin's worldview these actions make some forms of sense to him and. As said. From the outside perspective it might be reasonable to call it irrational. But that might not help the world in finding a way out. For that one needs to understand why it makes sense to Putin to act the way he does right now.