The general rotation involves using the main spells or weaponskills, like using Temper to buff the next weaponskills, and keeping their buffs active from their various abilities; the main ones being Bravery and Faith, with Concentration and Tranq enhancing them further.
Assuming all buffs are active, the best way to go through their rotation is to ensure they can use Temper to buff their higher-potency weaponskills, especially their dots, but needs to be timed at least two GCDs before they use Gavel to gain Faith, since entering Magic Stance would end the effect. Once they enter Magic Stance after using Gavel, they gain access to their tier 2 abilities, which would be Drain/Osmose, which in turn grants Deprotect Ready. Using Deprotect increases the damage of their next weaponskill and swaps to War Stance, which would ideally be used on using Smite as a finisher to end that branch of their general rotation and swapping over to Magic Stance, until the cooldown for Bravery comes up.
The main 5 spells and weaponskills are what their general rotation would be until their stance swaps, which functions primarily as a priority system. The three spells and weaponskills after those 5 function after their stance swaps.
A lot of what would make their utility stronger is knowing when to be in either stance, highly optimizing for each fight. Being in melee phase and using Spread won't consume the use of Spread when they use an auto-attack, so they could more readily Spread their various buffs like Barrier, Imperil, or Aura.
Optimizing their abilities and stances, and knowing the fights, are key to knowing when to most effectively
As for Battlemage and Spellblade stances, they're mostly a way to change the overall feel of the class. The idea is for them to use at least one GCD to incentivize remaining in one stance. Might be worth giving it a longer cast time. The damage buff is secondary; Battlemage's damage is meant to offset the lack of auto-attacks, and Spellblade's is meant to grant a bonus for remaining in melee range.
It is possible to change them both to instead affect auto-attacks and remove the damage buff, essentially granting them ranged auto-attacks in Battlemage, or adding int-scaling melee auto-attacks in Spellblade, but number-wise it would be better to grant a higher buff to Spellblade, since they'd be only in melee, and would have no ranged damage (as of now).