There are a finite number of 18-35 year old men.

Lmao, okay.

It's very much worth a read. The broad strokes are:

• There's a notable difference between the attack towards Kiev and the attack in the separatist regions (it also talks about attacks in southern Ukraine outside the separatist regions, but I think it says they're basically similar to the Kiev attack).
• The attack in the separatist regions were to hold territory with an amenable population. So you have a lot of troops, tons of artillery, and they dug in elaborate fortifications that they will actually stay and defend.
• The attack towards Kiev was an opportunistic raid to divert troops from the main thrust of the attack in thr separatist regions. The article talks about similar raids the Russian Empire did in the Napoleonic Wars, the Union calvalry did in the U.S. Civil War, pretty sure it mentions a Soviet one in WWII, etc. It involved much less artillery because it wasn't intended to hold ground and they wanted to avoid unnecessarily antagonizing civilians they didn't want to govern anyway.
• On that last point, the article also talks about how Russian missile strikes have largely avoided the most damaging civilian targets. It gives an example of striking an electrical substation that converts electricity into a type usable by trains instead of striking electrical infrastructure that is more general purpose (and would shut down broader civilian electricity, too).

The Kiev attack's goal appears to have been "disrupt, divert, and if you see opportunities, take them." I bet if the Ukrainian government had shown signs of folding or if the defense of Kiev had been weaker they would have pushed for more, but that didn't happen, the separatist regions were taken successfully, and the Russian Kiev column had no more reason to be there.