... But with popping inspiration right in the beginning of the fight to coincide with the power use relics and my Flyby, tanks aren't given enough ability to sustain threat through that kinda of dps output, especially with 2 sentinels hitting master strike during that phase.
Well there's your problem right there! If you're doing that much
DPS that early
in the fight, then you are begging
to pull aggro and wipe your raid.
That being said, we had to wait to use Inspiration and wait to use the power use relics until tanks could generate enough threat.
And that is exactly what you should do. You've got to wait until after the first tank swap to start that level of big burst damage. It's almost insanity to make your tanks keep aggro through all that your dps is doing.
There should be absolutely no concept of "too much dps." This should be something that's impossible to do, especially with a boss that has an enrage timer. The Zorn and Toth fight is way worse, I agree, but the Writhing Horror fight is bugged when you do too much damage too quickly and rip aggro off the tank.
First, if you're doing that much dps, you'll never hit the enrage timer to begin with. Second, while "You can never have too much dps" is as true a maxim that can ever be said, having too much dps at the wrong time during a fight can cause more harm than good in a split second. Third, if you're complaining this much about dps problems on the Writhing Horror, then I'm surprised that you're not complaining about dps problems on the tentacles in the first phase of the TFB fight. Oh wait, dps is most likely holding back so tanks don't lose aggro on the tentacles (assuming you've made it that far). Do the same on the WH and you won't have any problems.
And honestly, if you aren't encountering these issues, you might want to make sure your dps are doing the proper rotations. I've had to leave the flyby completely out of my rotation in order to not bug the fight. That is just not acceptable.
Most guilds DON'T encounter these problems because they HOLD BACK their dps until the appropriate moment. Perhaps, your dps might want to re-roll and learn to tank? Or read up on how threat works and how taunts work on threat? That way your dps can have a better understanding of the horrors you are putting your tanks through (and by extension your healers). Just a thought
Edit: Quick question. Is your raid running with 2 Jugg tanks? Cause Jugg tanks are still notorious for losing aggro.