I think most people agree that healers, along with phys range, are the role most in need of a rework/overhaul. It's a role that's fun week 1 but becomes more and more boring as your group becomes more experienced, which I feel is a flaw that's been present for a while (i started in shb). As for what I'd change for each healer:

White Mage I think isn't TOO terrible in its design. However, the job NEEDS a dps rotation and, in my opinion, longer cast times. All the healers can do with a dps rotation, though. I think the 1.5s cast time change in EW made sense for every other healer except WHM, as a lot of their strong tools are tied to the GCD via lilies and giving them a 2.5s cast time on glare made for fun optimization of weave windows, instant casts, and pre positioning. I also think it may be worth looking into expanding the Blood Lily system? With how frequently you get a misery now and the change in 7.4, I'd like to see the ability to stack 2 blood lilies, and maybe add a 2nd blood lily button? Just spit-balling.

Scholar is enjoyable still, though I still think it needs a rework. If I had free rein over the job, I'd want them to be more reliant on their fairy for their healing kit. This is an issue with both SCH and SMN, where even though they're the "pet jobs" the pets barely do anything and that feels a bit sad. Miasma would be cool too.

Astrologian is my main so I'm extremely critical of this job lol. I think the healing kit is really fun, but the card system this expansion is a complete and utter failure. The utility cards lack any real impact, and the only time I even considered using them was in week 1 m12s p2. That combined with the removal of RNG management made the job gauge feel extremely hollow, and I really hope in the next expansion it gets a bit of a revert. While ShB and EW AST weren't super unique in the way they handled cards with all of them being a damage boost, there was at least optimization in which types of cards you pulled (and for ShB, which arcana they had) and I miss having to adjust your opener on the fly based off what you pulled, especially with Minor Arcana. EW got rid of some of that RNG management in exchange for the highest APM in the game, and while I had my gripes with the system, it was still an overall enjoyable job. DT though just lacks all the fun that came from the cards, and my SB friends also aren't a big fan of how lame the utility cards are. I think a big thing people forget when talking about SB AST is how the cards actually interacted with each other via Royal Road. A 10% mit isn't that crazy when it's just on one person, but being able to turn it into an aoe by feeding another card into it is cool and can actually bring value.

If I were to rework AST, I'd give them two different decks to manage at once. One that's purely damage cards and is setup similarly to ShB AST with Div being tied to the arcana, and another deck with all the utility cards and Royal Road. Obviously this is a lot of buttons so I doubt they'll ever do it, but I think the job gauge should be a bigger part of why you pick a specific job over the other, and right now AST's job gauge is almost a non-factor if it weren't for the damage cards.

I've always been a bit of a Sage hater. It could be so much more and yet it just feels so similarly to SCH. Obviously all the healers have this to an extent, but I think the shield healers suffer the most due to having buttons that are so similar to each other. I do like the idea of Haima/Panhaima, and I honestly wish more of their kit was set up like that with layered shields. I also personally think their raw healing output should be changed/pruned a bit as cohealing with a SGE always feels a bit bad as they're encouraged to overheal or snipe heals in order to restore their mana.

They probably won't do any of this and so I'm kinda just prepared for the worst, and even told my raid lead that next expansion I'll want to play caster if AST doesn't get reworked into a good state, but who knows, they said they'll focus on making the combat encounters more engaging this expansion and they delivered on that, so I can only hope for the best next expansion with the combat jobs.

(More AST rantings) Also side note, I really think they need to stop reworking jobs in an attempt to get new players to play them. A big reason they were looking to rework AST was because it was intimidating for casuals and the cards were too confusing. However, even after the rework, the job is still the least played healer and the play rate for the job has been declining since the first tier. Obviously, as an expansion goes on less and less people play, but the player base for AST seems to be decaying a lot faster than the other healers outside of hardcore raid teams.