How serious is the Chinese military’s lack of experience?
Pretty serious, but probably not in the way you think.
For the shooty kind of experience, only Russia and Ukraine have it when it comes to open modern warfare. However, any competently managed army can gain such experience as they fight.
The experience China is lacking is logistics, moving people and things around so the army can keep functioning. The US military already mastered this ages ago and still practice it every day simply by having bases all around the world.
What’s the point of having a ton of tanks when you can’t get them enough ammo or spare parts to function? A big reason why the Axis lost World War 2 was because they couldn’t supply their forces. People like to talk about the U-boat campaign but very few remembered the far more successful US Navy blockade of Japan that shut down the flow of resources from Southeast Asia. Arguably, it was more decisive than the nukes in forcing the Japanese to surrender.
Similarly, people love to talk about Omaha beach and airborne landings on D-Day as well as all the battles but they rarely appreciate the far larger logistics effort to keep the forces in France supplied after they landed. To be fair, it wasn’t seen as a glamorous job either back then, with most of the truck driver jobs being given to Black soldiers. But without this, the army wouldn’t go anywhere or shoot anything.
If China wants to take Taiwan, they better be able to replicate this. Most countries today probably can’t. The US military had recent practice with this in Iraq and Afghanistan, where insurgents love to target supply convoys and other “soft” targets and where they had to resupply troops in hard-to-reach locations. That was likely more valuable to the institution than “infantry combat” sort of experience.