I was maining Monk and I will be switching over to Machinist.

If you're not part of a serious raid group, then it doesn't really matter what you choose, you have time to try them all and do whatever you like.

I've heard that the serious players are shooting to tackle the new endgame content with 2 weeks prep time. That goal should be attainable if you continue your main job from 50 and bring it to 60.
But starting a new job at 30 and taking it to 60 within 2 weeks sounds difficult and will require a lot of play time within a short period. We all know the tips and tricks to get to 50 pretty quick, but we don't really have an idea yet of what 50-60 will be like, so starting a new job will require double effort because you'd have to learn the new job AND the new content at the same time.