The FBI reports to the president. To run an investigation takes the approval of the hierarchy and a charge number. If an investigation was run without a charge number and approval then that would be a criminal act, for which the agent could be charged and imprisoned. I don’t see such approval ever being given.
The thing to do is to give the evidence to the opposition in Congress. (Which would also eventuate in criminal charges.)
Which is exactly why Trump was never going to have any problems from the .... Oh? Whuhuht?
Which would give the Republicans a golden opportunity to huff and puff, appear on all the right TV shows, and then do nothing.