Currently, he only seems interested in territories inhabited by ethnic Russians

Currently that is all he is openly or semi-openly taking action on. As for what he is interested in, he is on record as calling the collapse of the Soviet Union "a major geopolitical disaster of the century". It seems clear that he wants to restore all of that territory. All else is tactics.

I doubt he is much interested in taking, say, the Baltic republics or cis-Dniestr Moldova, as he would have to rule there with an iron fist on an uncooperative population.

Ruling with an iron fist on an "uncooperative" population (if they don't actively fight, what will their "uncooperation" get them?) is what Russia does. It did that through the years of the Soviet Union, and before that under the Tsars, when it was described (by the Soviets, no less) as "the prison of nations". Plus ça change, plus c'est le même chose.

Prediction: within five years, there will be separatist unrest in all of the places you mentioned, fanned (but not openly) by Putin.