I hope that the courts take Trump to task without even the slightest whiff of influence from Biden on the matter. Any suggestion of presidential interference will inflame the situation unnecessarily.
The President can’t direct the FBI etc. to investigate anybody (beyond making a complaint just like any citizen can), and they can’t direct them not to investigate anybody. He most certainly cannot direct the courts to do anything at all.
IMO, the best thing to do would be to set up an investigation by a special counsel, led by a moderate Republican and staffed by competent investigators without any consideration of their political affiliations (i.e. how this stuff is supposed to be done), and tell them to use their discretion to investigate anything that constitutes serious offences, lay criminal charges where appropriate, and make a public report. Biden should then pardon Trump for any of his stupid stuff that he did in office like obstructing justice, but let any financial/tax stuff go to trial and make him pay whatever fines are due. Trying to prosecute him for every dumb thing he did will be counterproductive, so just stick to the big stuff and settle for publicly showing that he’s a crook rather than insisting on jail time.