Technically, a healer's ability to drop heals on a target is counterbalanced by having an action less about damaging/interrupting the enemy directly and said heal being gated by its cooldown/recharge timer.
WHM's damage output is definitely respectable, but not on par with actual DPS or even some of the tanks because of Cure II + Aquaveil in their kit. Arguably the greatest gain here is being able to hold "Cure III ready" for the entire cooldown of Seraph Strike. Seraph Strike also providing 10% Protect I suppose, but you gotta dive an enemy for it. Then you have LB: Afflatus Purgation which has a bit of a regen component to it. That said, WHM healing is generally on the GCD.
AST however is being overly favourited here by the designer. For one, assuming it is used at the ideal time (at <= 50% HP), Aspected Benefic heals for 9000, with 2x 4000 heal following in the next 4-6 seconds (total 17000). Even if we only use one regen tick here, this already outperforms Cure II for the same CD timer, while also being instant rather than having cast time. Beyond this, using a charge of Doublecast you can fire off a second Aspected Benefic with the same base heal formula, but a 4000 HP shield (so essentially "healing" for 4000 yellow HP), going up to the same 13000 effective value as the first one with one regen tick - and the second heal isn't on GCD, it is off GCD; an ability.
Doublecast recently (patch 7.1) had the recharge timer dropped from 15s(?) to 12s, while Gravity II is sitting at 20s - so you have MANY opportunities to use it for Aspected Benefic throughout the match.
On top of that, you have a 50% change to draw a Lady of the Crowns for an AOE 6000 heal and 10% party protect, and a SCALING healing component on Macrocosmos, which is part of your bigger engagement tools (for damage). The heal starts at 5000 and gains 1/4th (25%) of the damage inflicted on a player into account for an additional heal. So even if you already trigger it at say 30000 dmg dealt, that is already a 12500 heal for that player and without a hard limt, this can only go higher as HP + MP + CDs allow. Triggering the heal also either happens through expiration or using the ability Microcosmos, so it wont even trigger the GCD.
And then, to top it off, they have Epicycle - which I suppose was a cool idea given you are now forced to run into the enemy to use Lord of the Crowns (where-as using The Balance was safe in your backline), but with an additional 6000 HP shield (formerly 8000), a return-to-teleporter (Retrograde) you have 10 seconds to use before it expires and using Epicycle doesn't even need a target unlike Seraph Strike (and Icarus on Sage). It simply does too much and has no inherrent risk (I do not count misplay / mispress of a button as a risk).
And the irony is their damage is still relatively solid compared to White Mage, while having WAY more healing and mitigation.
Personal conclusion: White Mage is strong, but fair - Astrologian is just absolutely busted for no good reason.
Here's what I'd change about Astrologian (not all at once, but ideas that could be mixed and matched):
- Aspected Benefic: Reduce the regen from 4000 to 3000 per tick, making it max out at 15000 total healing // 12000 after 1 regen tick at ideal HP use
- Aspected Benefic (Doublecast): Remove the heal scaling based on HP, so it is 6000 heal + 4000 shield // Experimental, but it is still oGCD, keep that in mind
- Lady of Crowns: Reduce the heal from 6000 to 4000 or split it into two regen ticks of 3000. // debatable suggestion given RNG nature
- Epicycle: Remove the shield entirely + reduce the time you have before Retrograde expires from 10s to 3-5s. // Keeps the burst mobility, nerfs the freedom of zipping back and forth
- Macrocosmos: Shorten the time of Microcosmos triggering to 8 seconds (parity with Riddle of Earth) // Keeps the theoretical maximum still high, but a nerf to the opportunity window
Personally I think changes to Aspected Benefic and Epicycle are the best picks here, as those are the painpoint of what makes AST go "from 0 to 100" at "no MP remaining" and then zipping away with Epicycle.