I don't think US invasions in Middle East can be compared to the Russo-Ukrainian war. In this case, it's developed nation vs developed nation, not super-developed nation vs semi-developed shithole. US could blitz in with elites against Iraqis because they knew that their military is bad; I mean, just look at the casualties on both sides.
In this case, Russia has to be careful. They know that Ukrainian society is as developed as their own, and it doesn't take some tactical genius to know that their military is probably like that too. That's why Russia is sending in weak units first; if they can figure out the max strength of the enemy using what is essentially cannon fodder, they can then figure out what the composition of their main invading force should be. That's assuming Ukrainians take the bait, and from the footage that has been gathered of Russian convoys being burnt and troops being killed (and photographs for those things), it seems like they're doing that.
EDIT: Figuring out the strength of the enemy... that's what undercover agents and spies are for. asghasfashdhsdfg I'm so stupid, I take what I said back. Shouldn't have written this after eating just around an hour ago. Perhaps Russian military is that mediocre...