Does job design even remain interesting outside of that initial period as a new player when you're learning how a game's job/class system works and what it tends to offer?

There comes a point when job design starts feeling very boring and mechanical because you know it well. Muscle memory and habit take over from conscience decision.

Rather than try to guess at what might make it more interesting, come up with examples from other games where it already does remain interesting year after year then identify the element that allows their design to remain interesting.

Chances are good it's a normal reaction to how systems work, whether in games or in life. Anything can end up boring once you do it enough.