Armchair general opinion - Ukraine are playing for time, why push too much when they can demoralise Russia with heavy losses (defending is always easier than attacking).
More time means more supply lines bringing weapons in, and more capability for long range attacks from HIMARS and their other mobile artillery. More training for their troops in general, more chance to get training on better NATO spec kit etc.
Pushing into Kherson could be very costly in terms of lives even if prepared to do it right now, it's better to force as many Russians to leave as possible by making re-supply hard. Winter is setting in as well in the not too distant future.
This is just my armchair general opinion though, I think Ukraine have mainly got a "hold the line" mentality right now.