Though I will disagree on what Putin will pursue next after he swallows Donbass; he won't go west immediately. You all may be underestimating the importance of the resource-rich and, in all cases except Uzbekistan and Turkmenistan, fairly pro-Moscow governments which will probably end up as part of Putin's "Union State" thing in Central Asia. If he's to annex Kazakhstan/Kyrgyzstan/Tajikistan he'll have to act before they fall entirely in China's sphere.
There is a complication there. A) Russia only borders on Kazakhstan. B) If the "polite people" show up in Kazakhstan they will be arrested and/or killed within hours. Kazakhstan is a proper dictatorship, if somewhat benevolent. And it has been given warning. Apparently, security services in northern Kazakhstan are super-vigilant already, taking care of shadowing visitors from Russia.
Isn't Nazarbayev going willing-ish-ly in exchange for getting to keep his control over Kazakhstan? It's already part of a customs union with Russia and they've repeatedly declared their intent to be one of the inaugural nations of the Eurasian Union when it forms in 2015 (with Russia and Belarus).
