Russia already had a majority control over Ukrainian oil interests because they have the bargaining power of being (and this is still true btw, during wartime) Ukriane's main supplier of natural gas. They could have, and it probably would have been smarter economically, for them to, bargain with the Ukrainian government, trading the political rights of the Donbass for control over the oil. But that is not what this war is about. This war is primarily about NATO, geo-political control, and the fact that it would have been massively unpopular and incredibly disruptive for Russia to give up the Donbass, and what they were doing was not a long-term solution as long as Ukraine continues to arm themselves and politically bang the drum for a full-scale invasion. Whether or not that invasion was imminent in 2022 is unclear, as the Kiev government was completely taken by surprise by the Russian invasion, but also had been making huge rhetorical speeches about retaking the Donbass regions. It's very unclear at the moment, and history may or may not provide clarification.

You aren't incorrect that capital is concentrated in the hands of a national bourgeoisie, but that an imperialist economic model does not strictly make. As @panopticon@hexbear.net pointed out, being imperialist is about separating a 'core' from a 'periphery' and treating them as exclusive zones for exploitation. The development of Crimea for the last decade has shown that that isn't what Russia does in areas they annex, they are simply incorporated into the 'core'. Now we can argue about if their exploitation of the Chechens is imperialism, but even then, Russia is almost always trying to be an honest broker in their deals (with the continued natural gas trading to Ukriane is evidence for). They are capitalists and exploiters of their own people, but imperialists is abit of a stretch, from a Marxist-Leninist definition.