Start all new jobs at lvl 30 or introduce new pre-requisite classes for them at lvl 1

[Kurando]

By level 50 its pretty obvious you've seen enough duties that you generally know the concept of what the 3 job roles do that you can figure it out.

Unless you've used the MSQ skip from MogStation in which case any expectation of skill is then thrown out of the window. It's only going to happen again with ShB, new players may get excited at Gunbreaker but want to skip straight to the new expac. They pick the new tank, run some duties and have no idea what to do.