I don't think that mainland China needs any sort of precedent over Taiwan should they decide on military action. The situation is completely different from Ukraine. The South China Sea is also on long-running dispute that predates the PRC (and dates from a time where all the neighbouring countries were Western colonies...) and what has been happening is more a policy of "fait accompli" by occupying unoccupied disputed islands first rather than an "invasion".

I don't know what is the thinking on Ukraine now in Beijing, but they were massively pissed off when Russia invaded because it has caused a lot of disruption to belt and road and to East-West relations in general.