Those 'channeled spells' you give as examples of 'its bad' are 'bad' because channeling them eats into your damage. Presumably (I haven't played ff7r) Soul Drain is a damaging skill. So, if it's 'channeled over X seconds' and does more damage than just using it then immediately clipping it with another GCD, then it will be held out for the duration of the channel. In fact, it's less punishing to use a channel than an actual cast time, because if you have to move 50% of the way into the duration, you lose the whole cast time and have nothing to show for it, but you still got half of the channel's effectiveness.
You want an example of a damaging channel, you've got Flamethrower. But that only gets used in AOE because of tuning. If it was 2000p over it's 10s duration, it'd get used on CD and people would complain about how they have to stand still for so long (but still use it because its the most damage)
And Umbral is not a 'building block', it's a polarity. Fire isn't made of Astral, or Umbral. Fire is like iron (an element of a different kind I guess), and Astral or Umbral are the magnetic polarities it gets pulled towards. Or we could look at it as a lot of different dualities. The game also addresses them as Dark and Light respectively. None of the six elements are made of 'Light', but they can be tilted towards that state. One that might make more sense, especially with Shadowbringers plot and such is, Entropy and Order. Being a practitioner of a school of magic that is literally 'order-aligned magic' would imply a degree of control over what makes reality, reality. Indeed, we saw what pure Order as a force can do, it erases everything and just leaves blank white nothing. But, I think that kind of aesthetic, of someone with that kind of power, is a bit too 'god-modey' to be messing with. Especially when WHM's opposite number, BLM, doesn't use 'Pure Astral' in the same way WHM does with Glare. Astral Fire and Umbral Ice are a thing, yes, but to be WHM-equivalent, BLM would have to shift over to using Xenoglossy in place of Fire4
Imagine if you will, since life itself is inherently entropic to some degree, that to properly represent the 'power' of pure Umbral, of it's stasis inducing effect, a White Mage has the ability to hit someone with a spell, and essentially disable 'life' in that person. They breathe, but oxygen no longer soaks into their blood. Their heart beats, but the blood does not circulate. And they're completely aware of this as they die, because the stasis didn't remove their ability to think, just their ability to live. Terrifying thought, right? But we don't get that represented, partially because morbid, mostly because it's very difficult to convey that concept via gameplay. So, instead we have glittery lights. Which is why (I think) some people preferred the 'down to earth' (heh) nature of the elemental spells. BLM has Fire, Thunder, Ice, and at higher levels, dabbles in 'pure Astral' with Xenoglossy/Foul. WHM is the foil to that, using Earth, Wind, Water, and at higher levels, dabbling in pure Umbral. If they had explained it, rather than just converting all the damage spells to lights, as that the WHM uses Umbral polarity as part of their spells in focused amounts to slow down the life processes of a wounded ally, so they have more time before the wound claims them (if they have been stabbed, using Umbral to slow their heart rate so less blood leaves through the wound buys them more time), that'd have been a completely logical and lorefriendly way to implement the Umbral side of things, I think. Or if they really wanted to make something of the damage side of things be 'Umbral-ized', just do it to the DOT. Dia has been a previous FF White Magic but I don't recall Glare being one. Was it in FF11 or something
4.6k characters ffs not again