Firstly, let me say that that’s a damn good take on the allagast MH. Devotions are beautifully done, and so too were the item choices with one sole exception - the belt.
Like it or not, storm shepherd belt is BiS here. You’re just going to need to make do without the skill points. It offers too big a damage boost to both TSS + SBoE.
I would also maybe try starfury amulet instead, but I doubt it’d make too big a difference.
Moving on, tether scales with casting speed, so you might want to 10/10 mental alacrity.
Lastly, I would try iskandra’s relic.
FYI - The biggest power of allagast set lies in deadly aim. I won’t go into the details here, but you can read my explanation here
EDIT: Welcome to the forums! Hope to see more quality builds from you. 