Returning Player Contemplating Classes

[Teiren]

Honestly, just don't overthink things. I say that from personal experience of having gone in circles in my head only to confirm the simplest answer was the correct one all along. I think the game is mostly deliberately designed like this. You can succeed at next to everything in this game by just doing the fundamentals correctly, and the fundamentals are honestly pretty simple when you just look at the Potency numbers and remember Damage over time Potency always ticks every 3 seconds. Every guide I've seen is naturally overwhelming to a new player because they go into all this min/max optimization stuff that isn't incorrect but also largely irrelevant to someone that just wants to play. Most of that optimization is written for people that have long since mastered the fundamentals and are looking for any other potential edge to learn and hone, so don't worry about that stuff. Just take it nice and slow.

That said, if you have specific questions, I'll be happy to answer if I remember to check back here. There are a couple other consistent posters here that give great, sensible advice. I will say this about tanking: The biggest breakthrough for me back then was realizing I don't need to attack the same target with all the hits of a weaponskill combo. Using Savage Blade/Skull Sunder on one enemy and then Rage of Halone/Butcher's Block on another one is much better than just using Flash/Overpower again, though Flash/Overpower should still always be your first action against probably all enemies and definitely if there's 3 or more.

One more thing: I said it's all simple, but it is true that the game leaves out vital details in some things, which is just stupid but that's how it is, so here's some useful data in case you're curious:
https://www.reddit.com/r/ffxiv/wiki/enmity This will show you how relatively useful the different tank weaponskills are in holding aggro on enemies.

As for Black Mage, I'll just spell out the numbers of Umbral Ice and Astral Fire:
Umbral Ice: Regenerate +30% MP/3 secs, -10% Fire spell damage, -50% MP cost for Fire spells
Umbral Ice II: Regenerate +45% MP/3 secs, -20% Fire spell damage, -75% MP cost for Fire spells
Umbral Ice III:Regenerate +60% MP/3 secs, -30% Fire spell damage, -75% MP cost for Fire spells, -50% Cast Time for Fire spells

Astral Fire: +40% Fire spell damage, -10% Ice spell damage, +100% MP cost for Fire spells, -50% MP cost for Ice spells, Impossible to self-regen MP
Astral Fire II: +60% Fire spell damage, -20% Ice spell damage, +100% MP cost for Fire spells, -75% MP cost for Ice spells, Impossible to self-regen MP
Astral Fire III:+80% Fire spell damage, -30% Ice spell damage, +100% MP cost for Fire spells, -75% MP cost for Ice spells, Impossible to self-regen MP, -50% Cast Time for Ice spells

What this all boils down to is exactly what Frizze laid out.