I don't entirely trust Biden when it comes to this, however, you can't really compare Taiwan to Hong Kong. To take Taiwan, China needs to annex an entirely separate country. They can do that through soft influence (pretty sure everyone agrees that's totally a failure), through economic means (Taiwan has been diversifying it's economy so not sure how great that option is anymore), or militarily. Even if other countries were to stay out of it, it would extremely risky for China. They very well could fail. Comparing Taiwan to Hong Kong in this instance really is kind of ludicrous.
It may be risky for China, but if you actually read Chinese media (or even the halfway house that is the SCMP), you can see to them it's not a question of IF but WHEN...
What downsides? Even ignoring the economic and political fallout (which could leave them entirely isolated on the world stage), they could _fail_ in their attempted invasion even if only going head to head with Taiwan. The result after both a successful and an unsuccessful invasion could very well be a collapse of the PRC. They certainly understand this. That's way they've simply used the threats as a tool of internal propaganda. They may one day go for it, but it is very far from risk-free.
Given the HKHRDA passed the Senate unanimously and in the House by 417-1,⁰ bipartisan consensus even ignoring your entirely unsourced claim that Biden is a Chinese puppet means that there is some will to resist this prospect.
The problem in the US is not intent but competence. Biden can be relied upon to not e.g. flip-flop as Trump did on North Korea or petulantly ask what the point of a long-term alliance is, and therefore is much better for those countries in Asia who want to resist China but want their major ally not to behave so erratically. In fact, Trump seemed to think of China as a counterparty with whom a ‘deal’ could be struck for his own political purposes:¹ he had no actual ideological objection to e.g. the camps in Xinjiang; Biden, like most members of the blob, at least pretends (probably even to himself!) to consistently and resolutely oppose that sort of thing (the obvious examples of e.g. support for Saudi Arabia notwithstanding), which means far more consistency. And consistency is good for alliances.