I don’t see why some don’t find the idea of Finn working sanitation while stationed at the base to be so unbelievable. Aren’t new recruits in real world militaries often tasked with menial jobs before they can rise through the ranks?
Do the Green Berets complete their Green Beret training and then get assigned to the Pentagon to take out the garbage?
Answer: No.
Who said he had completed his training while he was stationed there? And where was it established that he is the First Order equivalent of the Green Beret?