Details matter.
Ukraine worked to bog down Russia because of geography and the raw power balance. Ukraine had multiple NATO member states as direct land neighbours, allowing endless and interdiction free resupply. It’s geographical position also allows NATO assets to see deep into the battlefield from territorial airspace and waters. Russian raw hard power is also lacking compared to NATO, allowing NATO to have escalation dominance and the Russians being afraid to effectively challenge them even when they are openly acting as part of Ukrainian kill chains.
SK and Japan are physically cut off from the rest of the west by vast oceans and distances. They are also far smaller in terms of land area. Supplying them during war time will range from extremely challenging to effectively suicide missions.
Chinese hard power is also vastly greater than Russia’s, as well as SK and Japan. Chinese hard power is also far more comprehensive and complete, in that it can undertake the full range of modern combat operations, including SEAD and DEAD that the Russians just cannot really manage. That allows China to hit SK and Japan with its full military might and range of weapons. Simply put, there is no realistic prospect that they will be able to bog the PLA down. Especially if China takes SK and uses that as its springboard to take Japan rather than trying to island hop along the second island chain, which will be a far more difficult and costly campaign.
As for regional instability, that only holds true if stalemate can be achieved on the battlefield. If you cannot hold your line on the battlefield, you will just get steamrolled and peace and stability will return to the region after you have been conquered.