I disagree with the use of thrash a lot, because it will ruin your force management. However, I'm interested in your theory that force pulling immune targets will still increase threat. Do you have any evidence to prove this?
It does. Used it multiple times on boss-immune targets, and got aggro from it.
As far as thrash or no thrash goes - its impossible to have a "rotation" with thrash in it that also allows shock and wither on cooldown. However, force management in linked to shielding/parrying, and sometimes you'll do enough of it to keep up your force above that. When that happens you use thrash.
But I agree, a cost reduction on shock somewhere in the tree would be nice, since we do miss out on energize procs, but mostly the "proccing" shock off cd is a false bonus in longer fight, since your force cannot sustain it.