"The US will have to bootstrap a lot of manufacturing to replace Chinese labor."
They just have to move to cheaper countries to replace Chinese labor. One time upfront asset cost, but lower expense over time.
"US trade "only" accounts for about ~18% China's total trade"
Once China is not part of the trade flow between US and X country (Taiwan, South Korea, Japan, etc), the total trade for China will fall even more than 18%.
"they can still manufacture goods intended for anywhere else in the world there - just not for the US"
US is the largest consumer market in the world. Japan is second at 1/3 of the size of US. EU consumer market is mostly fragmented. There is just not that much singular consumer demand outside of US. Plus other countries are enacting tariffs on China as well - India for example.
"Especially other foreign companies operating in China will profit from reduced competition for Chinese labor / manufacturing, encouraging them to move more manufacturing there."
There are lots of countries that have way cheaper labor costs than China, there's no reason for the foreign companies to go there in the face of increasing tariffs. Nike is 30% in Vietnam. Samsung is producing more of its phones in Vietnam. Uniqlo increased its presence in Vietnam by 40%.
"US companies will only withdraw in part from from China, since they can still manufacture goods intended for anywhere else in the world there "
Again, tariffs and cheaper labor costs elsewhere will entice companies to move out of China entirely. Not to mention the threat of Chinese government takeover of assets, or capital outflow restriction.