You have a lot of faith in China's ability to operate without western money flowing in. You also way overrate how committed to Russia China really is. China is an ally of Russia and will do business with them when others wont but they're not going to stick their neck out for Russia.
Just because certain technological parts or devices are manufactured in China doesn't mean that China will just turn around and sell them to Russia. These factories are in bed with western companies. They're not going to kick out the company that buys 99% of what they produce just so they can sell some to Russia. And if the company that buys all of the production from these factories doesn't sell to Russia, China doesn't care.
So much of what Russia needs to operate flows through the west. Could Russia change that? Sure, but it would take decades to get to that point.
Whoops