Add "public release of the tax returns of Donald J. Trump, the Trump Organization, all Trump children and their spouses and any other place that Trump money might be hiding" to the list of official acts that Biden might take here.
Pretty much, he'd need to get a new Judiciary Act through Congress, and good luck with that one. FDR's idea to pack the court was a legitimate threat because he had both houses of Congress on his side. Biden, not so much.