Joe blanket pardoned Hunter for past crimes. There is no such pardon for potential future crimes not yet committed. Contempt or perjury is on the table.
Technically, you don't waive your right to invoke the Fifth by accepting a pardon. By removing your exposure to the pardoned crimes -- in Hunter's case, "anything that might have happened since Burisma" -- the pardon makes it impossible for you to incriminate yourself. Ergo, you have no basis for refusing to testify on the ground that you might incriminate yourself.
Joe blanket pardoned Hunter for past crimes. There is no such pardon for potential future crimes not yet committed. Contempt or perjury is on the table.