A LOT LESS THAN NOW.
Need I point you back to Sabezy's timeline of the Byregot fight's damage profile? The content has the same intended audience as the Halone fight, which already has something like 2x as much damage and is still clearable.
There will be people bored no matter what, but for people playing content designed for their skill level, the answer is a lot less. Ex 5 is an Extreme yet there's a point in that fight where there's something like 45-60 seconds with no unavoidable damage. You think that's a good idea for fight design?
The across the board argument is the problem. You would need a full on DPS role (not "DPS at home") rotation before many people would not be bored running casual content. That's always going to be true. The answer is simply to give people that are more hardcore more hardcore content to run instead for their farms. Even WoW has normal dungeons and LFR difficulty raids, and did even back when healers were designed to not use DPS abilities in groups (high Mana costs making them prohibitive).
Healing requirements in casual content could be 2x or even 3x what they are now and still be survivable with literal Medica 2 spam.
Lol, never heard this before, but I love it.
To be fair, most avoidable damage does something like 70-90% (depending on role) of a health bar's worth of damage, meaning not having people at 100% means any mistake is death. It also means that unavoidable raidwides (which do 50-80% depending on role and mitigation used) are also death if not healed. You have all the way until the next attack to get people to 100%, but if people aren't 100% (or close-ish to it) when that attack comes, people die. It's the way damage is stupidly tuned now to be infrequent but very very high instead of frequent but smaller in magnitude.
EDIT:
The internet either ate part of my post...
I...thank you.
Honestly, if we could free up 2 buttons, we could add Aero 3 and I'd be more or less content. One for Afflatus, one for Cure 1 -> either Cure 2 (lower MP so that WHM actually is "the efficient healer" again) or Esuna (as discussed before). Then have Aero 3 in there, upgrade to Banish if we need to keep the holy element theme for whatever reason, done. Everyone wins. I think.
Not saying that's the end-all-be-all, but...seems like an idea that could be worthwhile, anyway.
EDIT2:
Please no.
Not every Job needs an Oath/Beast/Blood/Dragon/Ninki/Kinki/Shroud/Battery/WH-BLMana/Faerie Gauge that goes from 0-100 and has a spender that costs 50. Please no. We need the ones that currently exist to have interesting uses before we add more. Beast/Blood/Dragoon/Ninki/and mostly Kinki already are almost identical (blow during burst, don't overcap, spenders cost 50 each) to the point they're extremely samey. Let's not get more of that. Find ways to make Faerie Gauge legitimately useful first.
.
A simple solution, if we REALLY want this, though:
Filler spam spell: Earth
DoT: Aero
AOE: Water
Under Presence of Mind, empowers all your damage spells:
Filler: Glare
DoT: Dia
AOE: Holy
...granted, this doesn't have a spot for Aero 3, but it is what it is. Make PoM into Requiescat/Manification where it uprates your spells to more advanced forms. That would already be more unique than a gauge with 50 point spenders.