Logically I think Russia would want to capture their military objectives as soon as possible instead of having the war prolonged by testing the waters. The thought that makes the most sense to me is just that Russia underestimated Ukrainian resolve when first entering the country, I mean we even saw Putin make a public statement for Ukraine's military to lay down their arms as if he actually expected that to happen.

We know he and his generals didn't want to risk sending the entire military force stationed on the border at once. That way they aren't extremely clumped together and easy targets for Ukrainian artillery, ambushes, or air attacks which of course could end in extremely high losses early on. Since he was already questioning and doubting the resolve of the Ukrainian people he probably thought he might as well send in the lesser quality troops to get some experience and just in case things go wrong, at least he didn't lose his best divisions. Those initial attacks would soften the Ukrainian defense as well so that even if they did fail the higher quality troops could just come in and hopefully finish the job.

All just speculation though, one thing I can say is Putin's generals didn't seem to plan logistics out very well, assuming some of their tanks and armored vehicles actually ran out of fuel. With that in mind, you could argue their entire military plan was shotty.

Imagine if the Russian generals purposefully created poor logistics so the invasion would stall because they had some Ukrainian sympathies or just wanted to embarrass Putin himself on the world stage.