What I would change about~

Black Mage

Astral Fire and Umbral Ice
A 13 second window in which you have to fit 9 seconds of casting, an additional 1.5-2.5seconds of casting to keep going.
A problem that is solved with a massive amount of SS.... Which you can not even reach yet. Granted, the more SS you have the easier this hurdle becomes and as a BLM you are going to stack that anyway.
What to change
A simple solution. Extend the timer by 2 seconds. This just gives more breathing room to the BLM.

MP
MP issues on BLM? As odd as it sounds, yes.
Due to specific problems within Umbral Ice and MP ticks, your MP for your next astral fire phase won't be enough to cast 6x Fire IV which is your optimal rotation. This isn't losing DPS to a mechanic. This is losing to luck, this is not a proc and it should not be luck dependent.
What to change
Increase BLM total MP by 1000.
Or have UI decrease MP cost of Ice and Thunder spells by a certain amount.
This would allow for a constantly optimal rotation without breaking the game and allowing the BLM to get additional casts he wasn't supposed to have. (Eg a 7th Fire IV in your normal rotation)

Utility
I personally think a pure DPS Job is a bad idea. It will result in the mess that is 3.XX MNK where it is not a desired job or a mess such as SAM where it outshines all DPS.
What to change
Have Leylines give the party a haste effect. 10% for 20 seconds maybe?

Potency
Give us back potency period.

QoL
Have between the lines replace Leylines when it's up.
Have Fire IV and Blizzard IV switch out each other depending on if you are in AF or UI.
QoL for Summoner
Have Akh Morn replace either Enkindle or Summon Bahamut or even Rouse.
Pets, especially Titan need a sort of Auto Regen.

I am of the mind that jobs should only be nerfed when they are ridiculous. And I don't see that with RDM. I see it is strong, yes but the other 2 casters are just too weak in comparison.
Nerfing RDM will also have others be sour.
I also think buffing Jobs won't make this game "soooo much easier". Omega Savage isn't out yet and they should adjust numbers anyway.