Totally agree with Nickler, when I did my U.K. TRE standards course with the CAA twenty years ago they were saying that it is far better to start as a TRI in the sim. Learn to instruct, and really learn all the emergencies and how to deal with them, safe in the back of a simulator with a freeze button before trying it on a dark winters night in an aeroplane with 200 passengers and a 250 hour trainee in the RHS and a 350 hr “safety pilot” on the jump seat.
Twenty years later I work for a major U.K. operator who still refuse to let someone be a TRI in the sim before they’ve been a line trainer, despite the fact that the core course is the same.