Russia did not need Donbass, it did not want to take it, and for 8 years it tried to resolve the issue through diplomacy within the framework of the Minsk agreements. And then it suddenly became clear that neither France, nor Germany, nor Ukraine were going to fulfill the obligations they had voluntarily assumed, but were simply playing for time so that Ukraine would prepare for a war with Russia. Well, I'm glad you got what you wanted, now don't complain. They did not want to give Donetsk and Lugansk a little more autonomy and the ability to speak their native language freely - so much the worse for you.
In the situation with the Donetsk and Lugansk regions of Ukraine, the task was somewhat different. Russia wanted to achieve extensive autonomy in these areas with its own authorities, law enforcement and judicial structures and then transfer the destroyed territory back to Ukraine, so that by representing this autonomy in the Verkhovna Rada of Ukraine, blocking Ukraine's entry into the EU and NATO. But the plan of Russia was not destined to come true.
In the future, there may be several state languages in Ukraine, but not the language of the occupier. It is already proposed to introduce, in addition to Ukrainian, English as the second state language. This will forever put an end to possible further Russian aggression on the basis of the Russian language and the Russian-speaking population. By the way, already now the population of Ukraine, including the eastern regions, is voluntarily trying to switch to an exclusively Ukrainian language so as not to experience another "Russian spring" anymore.
This form of government does not at all contradict the multinationality in this state. Now almost any state is multinational, because in open civilized states, thanks to the current technical progress, people of all races and nationalities move freely. And this does not mean at all that the representatives of all nations in each state must necessarily have separate elements of their sovereignty.
Although Russia is a federation according to the Constitution, in fact it is more suitable for the classification of a unitary state, since the nations and nationalities representing it do not have any real rights. Moreover, now Russia is more like a monarchy, which is single-handedly ruled by the monarch - Putin. He makes sole important decisions, and all other state democratic institutions are actually subordinate to him. The central government rests on repressive law enforcement agencies that suppress any dissent. So, in fact, only Putin and several other persons close to him made the decision to invade Ukraine militarily. And to go out into the streets with slogans for peace and against war, for this any citizen faces a long prison term.
But failures in the war against Ukraine inevitably cause centrifugal tendencies, and it is precisely this that Russia should be afraid of in the near future.