Considering Bolton was on twitter celebrating the death of Soleimani Im not sure that Bolton will hurt Trump if he testifies
Personally that's what I think too. Bolton will go in and have nothing to say, which is the reason why he is so willing to testify. The man is not stupid, he may not like Trump all that much but he is smart enough to know that he would persona-non-grata in the Republican party if he testified against Trump, and it's not like a Democrat President would ever appoint him to any position, he is arguable more of a paleo-con war hawk than just about any Republican. In fact the reason he left the Trump administration was he felt the President wasn't hawkish enough.