returning player, completely lost!

[Niwashi]

If you need to test drive your jobs again, I would recommend unlocking Palace of the Dead

PotD is pretty popular for leveling since it gives pretty good XP, but I wouldn't recommend it for learning (or re-learning) your jobs. I suppose it might work if you go in with a fixed group of friends that maintains the standard party mix, but if you're either going solo or queuing for a random group, it can be any mix of jobs, so you lack the standard responsibilities of your job's normal role within the party. (It's kind of like leveling by just FATE grinding in that regard, with the same drawbacks for learning your job.)

I think it'd be better to re-learn your jobs through dungeons that actually involve using the Tank/Healer/DPS roles. If you've been away a long time and need to go over the basics, you can start with low level dungeons where you only have a few skills to focus on and simple challenges. Then when you're comfortable with those move on to increasingly higher level ones that incorporate more of your skills. The level sync lets you kind of work your way through the leveling process over again but without needing to hit XP thresholds while doing so.

[Rongway]

PotD ... you lack the standard responsibilities of your job's normal role within the party

But what you don't lack is access to all your actions, so you can enter floor 51 solo and see what sort of panoply you're working with, read tooltips, arrange bars, and see if your bindings meet your personal expectations.

This makes it a superior playground to standard party content, especially for BRD and MCH, who without the Palace could spend days playing normal content, thinking they enjoy the job, only to find as they get closer to 60 that they don't enjoy post-50 BRD. Reading the action tooltips and experiencing their effects as an acting Lv60 character will give someone a basic idea of whether they like a job or not, without wasting leveling time that could have been applied to something else.

[HyoMinPark]

snip

I wasn't suggesting that OP use PotD as the sole method of learning. According to the OP, he's leveled up most his jobs to 50. Going into floors 51-60, or even just running through 1-50, may spark his memory of "Oh! That's right! I use this skill with this one, pop this buff before I use [skill]." I was thinking more so along the lines of triggering memory when it comes to the jobs.

The post above also makes a great point of being able to test drive jobs after level 60, since some of them change pretty drastically (BRD for example). Let the OP know if he wants to pursue a job to 60 now, later, or never.