No official act is overtly illegal. Taking bribes is not an official act or a fundamental job of the President, so Joe could still be prosecuted.
OTOH, giving a speech to rally supporters is a fundamental part of being a political leader. As is contesting elections that you suspect of fraud.
I was thinking Biden better hope he has immunity from destroying the country by allowing 20 million criminals from 150 countries to flood in to the US in violation of the constitution and his oath of office.