In nearly every conceivable way the Russians had the advantage over Ukraine
No plan (or perceived/asserted advantage) survives first contact with the enemy.
Russia simply did not actually reform anything regarding its military since the USSR collapse.
You don't throw (already strained) money at a (non-money based) problem to fix it, that just creates inflation (exact same thing as hype and delusion).
So from onset I saw no real advantage Russia had over Ukraine (if Ukraine buckled up and decided to fight, which it did).
Strategically, the population ratio is just 3:1 as well. Essentially in many ways Russia has overall picked a fight with a peer (that too one that knows you deep down psychologically too)....which is just a dumb idea.
Vauban proved long time ago that with proper defence, organisation and resistance in large enough landmass (tempered already from earlier wars) you need something like 10:1 for any chance of breakthrough given defenders advantage...w.r.t conventional warfare for any age going forward (assuming no big tech level disparity).
Or as invader (to surmount this matter) you need a massive tactical (and risky) innovation commensurate to the land, strategic depth and opponents positioning.... but again that's something Russia simply paid no attention to from its inherited soviet psyche and approach which are just not relevant at all to this scope of 21st century. Russia deeply misunderstood that it cannot cookie cutter, copy paste its approach to conflict....Ukraine is whole different level of opponent and it didnt recognise that.