Yes, you can control the RNG in the sense that you can get an Umbral Card whenever you want. You do not control which Umbral Card you get, which is why I said 1/3. Because if you want an Umbral Card, most of the time the one you want will be The Bole.
You can put the RNG into the dps increase cards in the form of melee (Astral) vs ranged (Umbral).
And you can put the consistency in support cards in the form of Minor Arcana.
So to put this as an analogy to Aetherflow: do we use this astral/umbral card to gain DPS? Or do we turn it into a mitigation card?
It's not quite the same kind of tradeoff, though. Lack of Aetherflow will lock the SCH out of its most powerful abilities, whereas AST will be just fine without the mitigation from the card.
Because AST's kit can currently handle anything without any support from cards, I don't see many scenarios where someone would choose to exercise this option. Or any choice where dps increase is an option.
I thought this could work when I wrote my previous post, but nah. I don't think anything will work as long as the support cards compete with dps increase cards. So my next idea would be to leave the dps increase buffs to divination only and make every Major Arcana a support card. It would essentially become its own minigame and it's purpose would be to make your life easier.
As for Divination, you can rework it into a tiered system that depends on the number of seals, rather than the type (e.g. 7%/8%/9%/10%). You have 4 draws between Divinations. Use them all and you get the 10%.
This is still lazy, though. It's just shifting the balancing work to the healing side of things. These card buffs will still have to be balanced with the other healers. I don't know ...
But I hope our exchange illustrates how difficult this is. Job design and balance is not as trivial as a large part of the population frequenting this forum would have you believe.