Of all the classes, I find leveling mesmer to 80 "the worst" followed by engineer. Once all the traits and gear are in place, everything is much better. So I too recommend using the insta-80 and then learning how to play by going back through the story, earlier zones, etc. as need...
of self-preservation. Though its melee weapons didn't have quite the punchiness of those in the sequel, the original Vermintide relied less on leveling up and opening boxes of slightly better gear. If that slow gear accrual turns you off Vermintide 2, this first game remains a blast today....