Honestly, I never talk about AST because (a) I don't feel super qualified to do so (some of you think I shouldn't on the other three, but I've played WHM for literally 9 years, SCH for 8, and SGE the entire time it's been in the game + it has a lot of SCH in it) and (b) am not the type of player it appeals to and so likely don't represent those people and don't want to pull a New SMN robbery of someone's existing playstyle they enjoy (when I say people shouldn't have things taken from them - to support keeping some Jobs simple - I actually mean this, and it's a principle I actually hold, which means it cuts both ways and I don't want to rob other people of things they enjoy, either)
But if I set those two things aside for a moment to "thought experiment" what I'd personally think could work for AST as the "buffing one" of the Healers:
I think the big problem with old AST wasn't the different buffs, it's that they were all competing for the same spot. Just like my problem with SCH's Energy Drain competing against Excog, Indom, and Soil as well as Lustrate (if it was ONLY competing with Lustrate, that wouldn't be an issue, hence my rework proposal to push ED to Faerie Gauge), I think the issue with old AST cards was that the clearly superior ones were competing for the "this is my one buff every 30 seconds" with the clearly situational or inferior ones. "Balance fishing" was a thing because it was just always, or nearly always, better. Likewise, dumping Bole was a thing because there were almost no situations you needed it, and you generally didn't have it when you needed it.
The second problem with AST, I think, though I will caveat this with "many people like this", is that it's far too busy during burst, and has far too many buttons. It and PLD are the most button bloat-y Jobs in the game, and that's before you consider things like adding macros. But not only does AST have more buttons, you have to cram a ton of them into a tight window, which also limits other ability uses. Like Lightspeed would be a great Triplecast-esque ability if you could use it when you felt like it during encounters, but many AST players feel obliged to use it during their burst because otherwise there's too little time to press all their buttons, along with target swapping between allies and the boss, in that window if you can't double weave and such.
And I also think that a problem is that for a "buff mage", they don't buff...often. I mean, sure, you can throw out three cards in a burst window along with Divination...but that's at the cost of only using 1 card in odd numbered minutes and only throwing out an average of one buff per 30 seconds. Contrast MMOs or games in general with full on Support/Synergist/Buffing as a role, where they are CONSTANTLY throwing out buffs. Where their "filler rotation" is throwing out buffs, not spamming a nuke button.
(A more minor/lesser issue is that AST has so little personal damage due to its balance around party buffs that it's a slog to play in Deep Dungeons and solo duties - not even boring, it's just SO SLOW compared to even SCH [which also has the benefit you don't have to worry much about your health since you can just shout "Eos, take the wheel!" for the most part.)
So if I was to propose fixes to AST, I'd do so to target those issues.
1) To the first, I'd propose a pretty extensive rework of the card system. At the risk of button bloat issues. I'd probably split the Major Arcana into two types - offensive buffs and defensive/situational buffs. The former would probably be like BRD's songs, just more powerful and single target. The 1/2/3 ratio between damage, crit, and direct hit. This keeps them distinct (so they don't "feel the same") while keeping them all relatively useful. For situational stuff, I'd probably put damage reduction, MP refresh, and likely movement speed for the other one. Call them Astral and Umbral, call them Lords and Ladies, doesn't matter, the two just need to be separated. The next change I would include is the ability to stock any card to use as a quick stock/quick play. So say I get the Bole. I don't need it right now, but I know a Tank Buster is coming up in 30 seconds or so. Maybe I stock that Bole so I can play it them to save me some other CD/heal cast, and I can keep drawing and playing that card type otherwise. And I'd probably bring back the "Deployment Tactics" effect of being able to make a given card AOE at a cost of reduced effectiveness or duration.
2a) I'd probably make card draws and plays GCDs. I know some people would hate this, which is why I hesitate to add it, but I think the big complaint about AST is its burst. While you could just give them a low CD and charges to Lightspeed to the point they play like SMN, at that point, you may as well make this change. It would also help with having to target swap so much when playing cards, since making them GCDs would give you a larger acceptable window to choose targets (while still being a max of 2.5). Note that this could be accomplished with only part of the mechanic being GCD (Draw, say, with Play still being oGCD), or both being GCDs.
2b) Of course, this all comes at the cost of even more buttons. There are different ways to deal with this sort of thing, like how SGE uses Eukrasia to get double duty out of buttons, and I'm not sure the best fit for AST. One thing might be to cut down some of their redundant buttons, or buttons that are already covered by cards. For example, you would have less need for Exaltation if The Bole is already Exaltation. Combining some lesser abilities could also help, such as the oft-suggested Trait to upgrade Benefic 1 into Benefic 2, and another could arguably be used for Helios (same argument as WHM with Cure1/2 and Medica 1/2). There also might be some consideration to weeding out some redundant and near-redundant oGCDs, as AST is hardly alone among Healer Jobs for having oGCDs that mirror effects of GCDs, or even other oGCDs, as well as some abilities that are extremely niche at this point, like 20 second Beacon of Light Synastry on a Job that tries to avoid using GCD single target heals in the first place.
...the thing is, AST has a similar problem to SCH in that it has some oGCDs that are redundant...but most of the buttons do feel distinct and unique. Neutral Sect and Collective Unconscious, for example, are fun and unique/interesting abilities, at least to me, even if you could argue they have some overlapping utility. Earthly Star is, at its core, a big AOE heal (that does some damage, but is planned around for healing), but it's interesting and forms the backbone of AST healing plans, so removing it because "You have Helios" seems unwise as well. So I'm not entirely sure where to "trim the fat", as it were, as there's not a ton of "fat" to trim, and other than Bole, none of the other cards really have an overlapping effect with oGCDs. Though one thing that could help is having things "trait" add to cards with levels, instead of cards being the same from the time you get them to level cap to infinity 10 years from now. Like Bole could start as just a 10% damage reduction for 8 seconds, and get an additional 500 potency cure on expiration at level 86 (when you get Exaltation), just as one example. Maybe the Expedient one (Arrow?) could have the 200 potency shield from Celestial Intersection added to it from level 74 on, etc.
It's kind of like I've long thought NIN Mudras - they technically allow a great number of abilities to be added without a single additional button on the hotbar due to being combinations of existing buttons (there could technically be up to 15 if order mattered. Leaning into cards with that, especially the situational cards, could be an interesting way to add "more abilities" (or more features, anyway) to AST over time while countering that.
3) With the above changes done, more buffs (rather than less) would be the remaining thing. This either means very short CDs for at least some buffs, or even some buffs with no CDs. Ideally, the "rotation" should be "refresh DoT, cycle buffs on party members, refresh DoT", with Malific filler being rarely, if ever, used outside of soloing, and sparse Gravity in AOE 4 mans (where you have less people to buff). Even with 20 sec buffs on the two types of Major Arcana, that would still be 6 per minute (3 of each type). Minor Arcana could be reworked to be HoT (Lady) and MP Refresh (Lord) or something, giving you another thing to upkeep. I don't think no CD would be that great (especially on the damage increase cards), since then gameplay would just devolve into spamming those on every party member over and over, but short CDs for high buff uptime and buffs being a the "filler" of the rotation would be a lot more engaging than Malific spam, at any rate.
[4] would be something like I've suggested before of buffs used on the AST by themselves having increased effects to make the solo stuff a bit less painful. Possibly using the GCD buffs boosts the next Malific to make them damage neutral or something, I dunno.
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Anyway, again, I don't tend to mention what I think of AST, as I'm not sure it would work well or not alienate existing AST people, but it would probably be something on the lines of the above.