IMO, Russia doesn't have the manpower to just throw bodies at a war like Ukraine, because Ukraine is too big, with too large a population, and too well-trained a military for that to work. Now if it was a lone Baltic state, maybe things would be different.
Where the Russians have succeeded, or had more success with the Zerg rush strategy, is more when an enemy is advancing deep into the Russian interior, which involves traversing very harsh terrain and very extreme climates, and then very loooooong supply lines, in which case unless the advancing enemy has a very serious logistical, manpower,, and/or technological advantage, they start running into trouble. But this time around, Russia is the aggressor, so they are fighting a dug-in opponent with much shorter supply lines.