Here's what China needs to do: publicly announce that it will increase its nuclear stockpile to at least 5000 warheads, and carry out that threat ASAP. Then immediately renounce adherence to no first use policy, and make it abundantly clear that China is willing to go to nuclear annihilation to recover Taiwan.
The only red line Americans understand is one that is backed up by the threat of guaranteed, utter, and unavoidable nuclear annihilation. This is why they have avoided a direct conflict with Russia, all the way through the Cold War to the collapse of the USSR to modern day Ukraine war.