Russia wants to keep Kaliningrad mainly because they want to keep on being able to access the world ocean from St Petersburg (and the nearby port of Ust-Luga) and Kaliningrad helps with that.

(Controlling Denmark would help tremendously, too, which is why Moscow once asked Berlin for Denmark during or right before WWII as a response to Berlin's asking Moscow for its consent for Berlin to take control of Romania or some such.)

And yes, I believe that continuing to be able to access the world ocean from St Petersburg has helped Russia in its war with Ukraine.

BTW, "China ends up in either nominal or practical control of almost all of [Russia]" strikes me as so unlikely as to not be worth thinking about.