I don't think Holy's cast time is an issue - it's working as intended. Holy has a stun while the other AoE damage spells for healers do not, and its very OP for that. It makes dungeon healing much clunkier, but ironically makes lilies better as you can use them to weave. If they reduced Holy's cast they would remove the stun and I think no one wants that.
WHM right now just has bizarre disparities with AST. Cure 2 costs 1000mp for an 800 potency heal... Benefic 2 costs 700 MP for an 800 potency heal. They're identical. In fact, AST has a shorter cast time - if anything Benefic 2 should cost more! It's like this for all WHM and AST's basic heals and it just looks insane, especially when AST has incredible MP recovery and doesn't need the reduced cost.
Even if AST didn't have Astrodyne, it would have Draw for 500 MP every 30 seconds. WHM has Thin Air for 400MP every 60 seconds, and it has nothing extra. Of course I hold a single Thin Air stack for raises or medicas, but in reality the vast VAST majority of Thin Air usage goes on cooldown on Glare. It's extremely poor. With this said, I don't have MP issues unless things go wrong... in which case my MP doesn't recover for 3 minutes, I simply don't have the capacity to regenerate mana faster than I spend it.
I feel like WHM's problems right now are just its kit doesn't fit together, and the current focus on making healing 'easy' while ignoring players who like to optimise and do high end content has left WHM in a very odd place. WHM feels fine to play casually, but if you are trying to optimise it feels awful. You actively DON'T want to use your lilies unless in downtime. The 1.5 cast time has done wonders for WHM, as it can now actually use previously unused abilities like Plenary Indulgence, but as lilies cannot be weaved and Misery is a huge loss it makes them vastly worse.
Liturgy is beautiful and theoretically powerful, but in practice there are very little places you can get its potential out, which makes using it feel bad. Because of its long cooldown and short use period, you can't even use it on all of these places. You pick a specific moment where it will get as much use as possible (P3S add phase) and then work around it (meaning I'm hilariously using it immediately at the start of the fight on the raidwide and... the tankbuster?). It just isn't satisfying and is very frustrating.