why aren't the Russians deploying some of the capabilities they have but so appeared to have held back?
Sounds like a rhetorical question... obviously they don't "have" any such super-capabilities. The retreat from Kyiv, mobilization in DNR/LNR, and a few thousand troops from Karabakh and Osetia is all they can afford to throw at one last push in Donbas.
I think Russia plans to conduct this operation with limited personnel, emphasizing the dominant superiority in military equipment. For the Russian army, there are no problems with logistics, military plants are operating normally, the state defense order has been 100% stable for many years, Russia is well prepared not only for economic sanctions, but also for military operations on the territory of Ukraine. It is naive to believe that Russia will run out of money when Europe alone buys gas from Russia for a billion dollars a day. It is even more naive to believe that Russia will run out of food and weapons. Ukraine has not even really been bombed yet. I think the blow in the Donbass will not be the last, but rather the first really strong one.
Other than that there's just nukes.
Putin has an unlimited mandate for this operation, including the ability to make the entire territory of Ukraine west of the Dnieper uninhabitable for the next hundred years. The people of Russia will not forgive him only for abandoning his originally declared goals, and Putin knows this. But the mass genocide of Ukrainians due to a nuclear strike is not consistent with the current strategy of minimizing civilian casualties, so this alignment will be a technical victory for Russia, but an actual defeat for Putin, with long-term reputational losses in the eyes of history.
I see. Look, you have to put "tactical" before saying that type of stuff but it sound better. For example "Putin's army made a tactical retreat from Kyiv". See... that does not sound like a defeat now.
There is a saying in Russia, "war is bullshit, maneuvers are the main thing." The Ukrainian army is actually immobilized by the lack of fuel and air control, and is deprived of the ability to freely maneuver, while the Russian army is mobile and can create a local numerical advantage in different parts of the front. I think the tactics of the Russian generals is to deprive the army of the Armed Forces of Ukraine of general coordination and divide it into separate groups, this elephant needs to be eaten in parts.
Ukraine will be rebuilt. Who will rebuild Putin's Russia prestige and stance in the world?
Your concern for the prestige of Russia is very touching, thank you.