According to FFLogs, the vast majority of uploaded logs for the extreme trials contain WHM, it being the second most popular job after Reaper. Just because it hasn’t been your experience doesn’t mean it hasn’t been
the experience.
Anyhow back to White Mage-
Your suggestion for Thin Air, it giving multiple free spells, is probably all that’s required to bring WHM’s mana economy back in line. It’s a change I’ve been hoping for too. Though I imagine the current intention was to prevent people from getting multiple free raises under thin air, so it could be altered to only refund a specific portion of mana (or have Raise consume all stacks) rather than flat free costs.
My thoughts are that Misery should cost 2 blood Lily stacks instead of 3 (though you can still bank 3).
This would keep Misery at a damage loss (albeit only 30p over 3 GCDs) in a vacuum, but placing Misery inside most raid buffs would net an overall damage gain. It would also be a minor buff to WHM’s mana economy, as it means more Misery casts overall.
The cleave damage would need to be adjusted to suit, but that’s a minor issue overall.
As for Liturgy of the Bell, I think it would be worth taking notes from Sage’s Panhaima.
Have it apply 5 stacks of the healing effect to each member individually, which are consumed for healing when struck.