If they don’t testify, they can put people in contempt of congress which can be enforced by the Sergeant at Arms. This is something that did happen to Steve Bannon when he refuesed to testify for the Jan 6th committee. Not just a hypothetical power

The Sergeant at Arms is part of the house, not the executive

If Trump tries to illegally dismissing congress, they could just still meet and direct the Sergeant at Arms anyway