Somebody I know, did selection back in April/mid May as TRE/TRI. He said the contract is for 12/24 month for the trainers.
Everyone else will get permanent contract form the beginning but, conditions are terrible. As Cpt you are lucky to earn 4500/5000€ net. As trainer completely different story. You do need to speak German tough. He was supposed to start ASAP but not heard ever since.
Take care