[citation needed]. A decapitation strike doesn't just remove the leader, it needs to remove the whole (or a significant part) of the government. Additionally Russia likely wanted to replace it with a puppet.

It likely wasn't the primary objective, but most likely was an important one.

So they left a strong wartime leader in place for their enemy for their upcoming war? Why? Since you’re so sure about intelligence capabilities, why couldn’t the West have known and taken precautions?

There were multiple assassination attempts at Zelensky, they just failed. Allegedly he was even protected at some point by foreign commandos (possibly French), but this is still rumours. There were also kill lists found when the russian army retreated from Kyiv, clearly aiming at killing the "upper classes" (see https://en.wikipedia.org/wiki/Katyn_massacre for a historic example). BTW, this is what "de-nazification" actually means - purging the elites of their western, meaning "nazi", influences. After all the great nation of Russia is never attacking, always defending, this time again from the evils of the "collective west" meaning "nazi".

Russia withdrew because it turned out that their forces are (or at least were) utterly incompetent, and they just cannot continue. They got beaten up. I wish they stayed longer to lose more troops and equipment.