I am in the process of getting trims sorted for the carpet fitters, we have 20mm thick engineered wood floor joining with a room we will carpet. What is the best trim to use? I would think a Z bar but what depth should it be if a Z bar is the correct trim.

We had the same dilemma looked at various solutions. In the end we got 20mm oak thresholds made, that just stud proud of the 19mm thick wood floor.  Screwed down and pelleted.  Then the carpet fitters just laid up to it as the would any other area.  Small run of oak flavoured silicon between the threshold and timber floor is enough.