I usually use one Thrash/Double Strike after the Discharge and a 2nd one if it doesn't proc the shock, it is possible to lose aggro if you wait for shock to be ready by its natural cooldown. That's usually one of the rare spots for Thrash, I also still try to proc Energize for the 2nd charge on Recklessness like a total noob but I reckon it's better used on Wither.
Taunt debuff should be on the target still when Shock comes off CD, if you don't taunt too early.
I find the biggest risk of losing threat is mid-channel on the first Lightning without Recklessness.
Thrash early is a situational thing. If you're doing a lot of dodging/shielding/parrying, it can work out and you'll get a little boost from Recklessness + Energize. It can also leave you empty on Force and doing a lot of Saber Striking after your first FL to build your pool back up. Plus, in terms of overall benefit, having Recklessness active for FL rather than Shock is superior, so risking both charges on a Chain Shock is not ideal if you can help it.