When did voyager go wrong?
For me, they started to lose their way in season three
and the tried and tested, formulaic trek stories were starting to assert themselves after some good set ups had been basically abandoned (i guess they felt they'd done all they could with the Maquis and the Kazon had been found wanting as regular baddies by that point)
So yeah, it became a little more formulaic after that but the final straw was when Janeway completely lost her marbles in Scorpion
and established herself as an erratic captain (she was just about getting away with it before then) and Chakotay was establishing himself as someone that didn't have the balls to stop her
After that it was a fun romp but i think we all know it could have been so much more