At risk of second-guessing Russian General Staff, to whom I (and likely anyone here) have zero exposure, I would explain this as an example of one of Clausewitz's 9 Principles of War: Economy of Force.
Economy of Force is just that - economical. Use the least amount of force to fully accomplish a task, leaving the remainder of the force to attend other essential tasks (like 11 time zones, a northern border in the Baltics and now Scandinavia, an isolated Kaliningrad, countless NATO hassles and even nuke provocations, air and sea ops on two of the world's largest oceans, flare-ups in Armenia/Azerbaijan, Ickystan squabbles, color revolutions in your neighbors like Kazakhstan and Belarus, changing/improving relations with a new key ally, and the few foreign bases Russia maintains). Russia MUST also maintain a sufficient force that is not decisively engaged - to deter NATO. If they go all in on Ukraine, then you can bet NATO will intensify all other hostile actions. Russia may have a big Army but attacking at parity is nuts - unless you have other things to do, and you believe a parity attack is enough to get the job done. In fact, a parity attack might be your only choice, as you believe you have a moral duty to protect your "kin and kith," as Putin just called the people of Donbass/Novorossiya.
This fight is far from over. Russia still maintains a 1000 km front in Ukraine (the distance from New York to Charlotte, NC). They lost the exposed northern tail of the front, fighting a retrograde battle with a thin ring of second-echelon guys. Now they are out of Kharkiv vicinity, but they have a more natural obstacle to anchor along (a river). They lost a little on the Kherson front, but Ukraine's counteroffensive appears to have stalled for now and Russian artillery continues their mission.
Russia is about to consolidate their success in Luhansk, Donetsk, Kherson, and parts of Zaporizhia. Investors say: "it's never wrong to take profit." Russia is taking profit for what they can take, now. They are taking what they believe they can secure, for now, and when they believe the time is right, they will make another push for Odessa and points afar. A lot of dynamics "get a vote," including Ukraine, NATO, economics, and winter.