Without a binding constitution, there is no formal legal basis for a polity and people are free to do as they wish. E.g., join the Russian Federation or constitute themselves as an independent republic.
Even if that was the case, how does that justify Russia's invasion? Russia has made massive changes to its constitution, doesn't that inherently weaken Putin's right to rule by your logic?
You're starting from the fact that Russia is more powerful than Ukraine and working backwards to find justifications for genocide.
They formed and announced a defense pact with the independent Donbas republics, which Ukraine then continued to attack (shelling civilian areas, by the way). So Russia responded militarily in order to provide for the security of the republics in accordance with that agreement. They have not committed genocide whatsoever.
Has Russia made changes to its constitution illegally? That would be news to me. There is of course a strong argument that the dissolution of the Soviet Union in 1991 was not constitutional based on the results of the referendum.
