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.
Other than that there's just nukes.