You will still run out of mana using Thunder.

Here are the abilities you should remove from your rotation, in the order that you need:

Miasma II (stop using this if you were using it with contagion, for AoE, it is still OK)
Ruin II (yes, even for off GCDs. If you need to move, just DON'T CAST)
Thunder (yeap)
Shadowflare (at 2+ targets, keep it up still)
Fester (use energy drain instead)