Sage improvements: Kadia Heal/Shield Mechanic

[NegativeS]

So you want only 1 job in the game can switch from pure to shield? I mean.. that's why it was OP
Because it was the only job have it.

And I think AST and SGE have very different kit

In my experience, AST is/was OP because of its various buffs and numerous high potency delayed heals. Not because it could moonlight as a barrier healer when in a Full Party with a WHM.

Keep in mind, by the way, AST was locked into its sect during combat, you had to commit to whether you wanted regen or shields beforehand. What you suggest might be a little much.