What is your intention with each job?
White Mage is the baseline healer, and quite frankly the "best designed" as far as job identity. It has a unique identity as the healer who is not punished for overhealing or using their GCD healing because of the lily system, which makes it very friendly for new healers. For this reason, it should remain largely the same outside of its basic damage combo. Any changes should be done carefully as the job fits its niche well and allows for a smooth healing experience. I want to preserve its more forgiving playstyle, while giving white mages something to do in their downtime besides just Glare.
Scholar needs its identity back. A tactical, DoT based healer. Quite frankly, it's time to bring old Scholar back. The changes here would raise the skill ceiling of Scholar's damage with its old DoT kit and add some support options to lean into their "battle strategist" fantasy, and offer more variety in their gameplay outside of floor smacking. Scholar's changes seek to add flavor that has been lost ever since StB.
Astrologian is lacking in meaningful support, despite that being its niche. The RNG in the job feels "empty", as Astrodyne is negligible, Crown Play is just fishing for lords, and the cards all do the same thing. Major changes are needed for its core mechanics, but overall it provides a solid baseline of what a fast-paced higher complexity job can be. I would like to avoid too much additional button bloat. Changes here will be extensive but the general idea is, provide more meaningful fun, variety, and synergy between its core supportive mechanics, and give it a little extra something to do when it has downtime to malefic spam. Also, bring back some lost time mage aspects to the job.
Sage is currently a bit anemic. Its niche as the "vampiric" Healer-DPS feels underdeveloped. It is too close to current scholar and rarely has a use for shields despite being a "shield-healer", which leads Toxicon to feel underwhelming outside of being a mobility option. I would like to focus more on Kardia as a base mechanic for its healing. I'd also like to give Pepsis more utility and usability, because as it stands now, it's just a forgettable and generally unnecessary emergency button. The fact that it destroys your shields for potential toxicon stacks is an interesting but wasted opportunity, so I'd like that to be more core to the gameplay.
To give a good balance of overall job complexity, I would suggest the overall order of lowest to highest skill ceiling of all of healer jobs should go:
White Mage > Sage > Scholar > Astro
Keep in mind, this is assuming you are playing to optimize. I don't want the base skill floor of any job to become so complicated or difficult that people can't clear the MSQ/standard roulettes with it.