Disclaimer: I had to write this in three separate sessions due to going on and off my computer, I hope the red string of the content isn't all over the place.
To start off with, this is not a call to buff or nerf either sects. Personally, I'm content with the sects as they are and don't really care of the fact it's inferior to Scholar for it's role. However, hopefully it'll shed some light on why Astrologian can never be balanced properly as it is right now. Warning: There will be no tldr.
There are Astrologians out there not being content how Nocturnal Sect cannot rival Diurnal Sect for it's intended purpose. With the current meta, a Diurnal Sect can take over a White Mage's job in a raid, but Nocturnal Sect has some... issues, if I were to describe it. These issues are caused by the Astrologian kit itself.
So let's assume Astrologian in Nocturnal Sect somehow gets mitigation on par with a Scholar: Deployable aspected benefic and 15% disable for 10 seconds, ignoring eye for an eye for now. Even with the addition of a (periodically) deployable shield for 341 potency and a 15% damage reduction on the source, it'll still miss a vital piece that Scholars does possess and, to an extend, even White Mages do: The ability to heal while being in Cleric's Stance. I'm not going to discuss the healer DPS meta here, it's a thing and that's a fact. Those who want to discuss otherwise will have to start their own topic. The fairy's embrace is good for half a physick every three seconds (200 potency/3 seconds) and Regen is good for 150 potency every 3 seconds. Astrologian's healing over time ability is locked in Diurnal Sect, something Nocturnal Sect has no access for. This does not only result into a rather tenacious task to constantly turn Cleric's Stance on and off to achieve something similar. It is also very resource heavy (on both healers) to do so:
Regen is worth 1050 potency for 618 MP every 21 seconds.
A fairy has a one time cost of 1768 MP, of which is no issue when summoned before the start of battle. In 21 seconds a fairy's healing would be worth 1400 potency.
Astrologian would need to cast a Benefic and Benefic II to match a Regen in potency every 21 seconds. Which combined together costs 1148 MP
Then there's the thing that Astrologian's MP recovery kit. It's not too much of an issue with the fact that Astrologians only have Luminiferous Aether at their disposal. Their healing kit has a significantly lower MP cost for it's potency, after all:
Physick/Cure vs Benefic: 442 MP / 353 MP
Cure II vs Benefic II: 884 MP / 795 MP
Adloquium vs Aspected Benefic: 1326 MP / 707 MP
Medica vs Helios: 1237 MP / 1060 MP
Only exception:
Succor vs Aspected Helios: both 1326 MP
This would be another issue. While Astrologians do not get that same 10% MP recovery every 90 seconds like White Mages do, the reduced MP cost makes them perfectly capable as a main healer in a group. The damage spells, however, aren't as cheap in comparison. Combust and Combust II are only periodically applied, which are both adequate for it's MP cost:
Combust and Bio: both 240 potency for 353 MP
Combust II: 450 potency for 618 MP or 0.73 potency/MP
Bio II: 350 potency for 530 MP or 0.66 potency/MP
Combust II, while being the most potent DoT for it's cost and GCD, does not consume a lot of GCDs. The main issue would be Malefic II. It costs the same as Broil and Stone III. While it's more potent than Broil and slightly less than Stone III, it's MP cost is rather high in comparison with it's healing spells. This would bring us back to Luminiferous Aether: While it's good enough for the role as main healer, it's insufficient to support the MP consumption as supportive healer (or off-healer as some call it)
Even if Astrologian would get a 15% disable and a deployable Aspected Benefic, it's still very resource and effort straining compared to a Scholar and even a White Mage. With astrologian as it is now, it's impossible to balance either: You can't take away the regen aspect of Diurnal Sect, but this is exactly what Nocturnal Sect would need.
So what if Astrologian gets some flying moon with a face that can somehow magically heal things just like a fairy? You could consider this as a "solution". The whole issue about Nocturnal Sect being unable to compete with a Scholar in the mitigation department still stands.
Then what if the deployable aspected benefic and/or 15% disable are there as well? When this happens, you'll get a Scholar with White Mage raw healing capabilities. Imagine a Scholar with Cure II, Medica, and Divine Seal while having some unique abilities of their own. Astrologian in Nocturnal Sect would simply become a Scholar without it's weakness in GCD efficiency in healing.
If a 15% disable and maybe deployable aspected benefic would be a thing, why wouldn't this also be available for Diurnal Sect? It wouldn't make much sense that one sect gains benefits on some spells and/or abilities, rather than a variation. For example: 15% disable in nocturnal sect, 10% disable in diurnal sect. Why would this be a thing? Just simply to make Nocturnal more viable? With this in mind, the first ability that suffered from this decision is Collective Unconsciousness. It's both a good move from Square-Enix, but also a bad one. Collective Unconsciousness used to have effects depending on the sect. Neither were really good on it's own, sadly. And the part you actually want in the particular sect were locked in the other: Diurnal could have used the 10% mitigation and Nocturnal could have used the Wheel of Fortune effect - Remember the lack of sustained healing during Cleric's Stance I mentioned before?
Now that the effects are combined, it's good for both sects. Which is a good thing. But it also reduced the amount of abilities/spells depending on the sect from three to two. Effectively diminishing the differences between both sects. With this, it wouldn't be possible, with Astrologian as it is, to have Nocturnal Sect gain some sort of sustained healing through HoT. Simply because Diurnal wouldn't have much else to get aside from stealing the shielding effect from Nocturnal.
If the whole moon/sun-like fairy was a thing, then Diurnal would get something similar and Diurnal would simply become a White Mage with a fairy.
With Astrologian as it is, it's nearly impossible to change or add any existing abilities/spells to make Nocturnal more "appealing". There was a topic out there that proposed changing Time Dilation to "space dilation", some Deployment Tactics equivalent. This would lead to other issues: If it also affect cards, Enhanced/extended Balance/Arrow spread over the group would simply be too good. If it doesn't affect the cards, you're missing out on an extra 15 seconds buff time on the precious cards you're drawing. Also the only ability that allows Enhanced effects rival Extended (+50% effect vs +100% duration). For whoever wants to argue about this, you can consult this topic.
Of course, Square-Enix could just pull a black mage on us and increase the flat potencies. But would that really solve anything? Remember what the issues were with Nocturnal Sect:
Damage spells are nowhere as MP efficient as the healing kit for it's available MP to use with each encounter.
No continuous sustained healing available and it's alternative is extremely resource heavy.
Being on par with Scholar in Mitigation would make it a Scholar with White Mage (direct) healing spells and abilities.
To conclude, unless Square-Enix decides to make certain effects only available on Nocturnal Sect and have no variation of it in Diurnal Sect, Nocturnal Sect Astrologians will never be able to completely fulfill the role Scholars have with the current meta. Even if they did, is it really something we'd want? Imagine Aspected Benefic with no HoT in Diurnal, just flat 250 potency healing while it has a 130% shielding in Nocturnal. We'd simply shove the issue from one problem to another. Personally, I've accepted the fact that Nocturnal Sect will never be able to take a Scholar's role as supportive healer and I'm fine with that. If it wasn't the case, whm/ast would be more viable than ast/sch or even whm/sch if Diurnal Sect wasn't as it is right now. If either sects were too good, it would simply overshadow another healer and the weakest link would go to a job as whole, rather than an aspect of it. Nocturnal sect does have it's uses and is by no means completely not viable. It's merely sub-optimal. Much like having a White Mage as a supportive healer or a Scholar as a main healer.